Rebecca Dekker


The Evidence on: Group B Strep

Updated on February 6, 2023, and originally written June 6, 2014 by Rebecca Dekker, PhD, RN

In some countries, pregnant people are tested for Group B Streptococcus (Group B Strep or GBS) bacteria towards the end of pregnancy. If a pregnant person is a carrier of Group B Strep (sometimes called “colonized with GBS”) and not treated with appropriate antibiotics during labor, then there is a 1-2% chance that their baby will develop early GBS disease.

Early GBS disease is a serious illness in the first week of life that can result in a long and expensive stay in a Newborn Intensive Care Unit (NICU) (Steer et al. 2020). Group B Strep was first recognized as a widespread threat to newborns in the early 1970’s. At that time, 1.7 of every 1,000 infants born in the U.S. had early GBS disease (CDC 2010). Today, because of a major public health campaign called “universal screening,” early GBS disease is much rarer in U.S.-born infants, but it is still a leading cause of newborn illness and death in other places around the world.

The topic of Group B Strep can be controversial. In high-resource countries where some people face excessive medical care and overuse of interventions, using preventive antibiotics for the one-third of people who have a positive test—when it will not lead to illness for 98-99% of their infants—can be seen as unnecessary by some. They may have legitimate concerns about the overuse of antibiotics and the negative effect of antibiotics on a baby’s microbiome.

Meanwhile, some health care workers may get frustrated when parents who are GBS positive decline antibiotics, because if one facility has thousands of births per year, a 1-2% chance (which seems low to individual parents) could translate into dozens of sick babies who need intensive care.

In other places around the world (sometimes even within high-resource countries), marginalized groups of people are not provided basic health care options, such as accurate testing for Group B Strep, IV antibiotics, or medical help when they or their infants experience serious illness or complications. In those places, GBS is still a leading cause of newborn death.

So, what is Group B Strep, and why do health care providers worry about newborn GBS disease? Does the evidence support testing for GBS and giving IV antibiotics during labor to prevent newborn infections? What are the benefits and risks of IV antibiotics? What is the microbiome, and how is it impacted by antibiotics given during labor? Are there any alternatives to GBS testing or treatment?

In this updated version of the Evidence Based Birth® Signature Article, we provide a global perspective on Group B Strep, as well as an explanation of how GBS and antibiotics relate to the microbiome. As a content note, we will discuss stillbirth, infant death, critical illness of newborns, and disparities in access to health care.

What is Group B Strep?

Group B Streptococcus is a type of bacteria that naturally lives in the large intestine and can migrate down to the rectum, vagina, and urinary tract, usually without causing illness. All around the world, about 18% of pregnant people carry GBS in their bodies (Russell et al. 2017). However, carrier rates can be as low as 8%, or as high as 35%, depending on the part of the world that you live in and how common Group B Strep is in the bodies of people living there.

During most of your life, if you carry Group B Strep, it is considered physiological, or normal (ACOG 2019). However, GBS can cause infections during time periods when your immune system is not functioning at its highest, such as when you’re very young, or if you have a chronic illness, or when you are very old (Steer et al. 2020). These infections are called GBS disease.

In pregnancy, GBS can sometimes lead to urinary tract infections (UTIs), preterm birth, and stillbirth. However, it is somewhat uncommon for GBS to cause infections in pregnant people—it is the cause of only 1-2% of UTIs in pregnancy, and only about 1-2% of stillbirths (Steer et al. 2020).

For newborn babies whose immune system is not yet mature, GBS can cause a serious disease that usually requires a stay in the NICU (Puopolo et al. 2019).

There are two main types of GBS disease in newborns: early onset GBS disease (< 7 days of life) and late onset GBS disease (1 week through 3 months). Some newborns can also become carriers of GBS as well, but not develop sickness. In this article, we will be focusing on early onset GBS disease.

To prevent early GBS disease in newborns, countries around the world generally choose one of two approaches:

  1. The “universal screening approach.” Screen all pregnant people for GBS at 35-37 weeks (in the U.S. this has been changed to 36-37 weeks) and treat everyone who tests positive with appropriate antibiotics during labor. This is the method that is recommended by the World Health Organization and currently used in 60 countries including the U.S., Canada, Mexico, Brazil, Chile, Argentina, Uruguay, France, Germany, Spain, Australia, Portugal, Iran, Oman, the United Arab Emirates, and Japan (Le Doare et al. 2017).
  2. The “other risk factor approach.” Do not screen for GBS. Instead, treat laboring people with antibiotics if they have one or more of these other risk factors:
  • GBS in the urine at any point in pregnancy.
  • Previously gave birth to an infant with early GBS disease.
  • Preterm labor.
  • Fever during labor.
  • Water has been broken for more than 18 hours.

Note: The specific risk factors that are chosen may vary slightly from country to country. This method is currently used in 25 countries including the United Kingdom, Ireland, the Netherlands, Norway, Sweden, Finland, Iceland, Saudi Arabia, Tanzania, South Africa, India, Bangladesh, Thailand, the Philippines, and New Zealand (Le Doare et al. 2017).

Later in this article, we will discuss which approach is most supported by evidence—the universal screening approach, or the other risk factor approach. But first, let’s describe the microbiome and how it works in the human body.

What is the microbiome?

The microbiome is the ecosystem of trillions of microbes (bacteria, fungi, protozoa, and viruses) that live and co-exist with you in certain places in your body—such as your skin, gut (intestines, rectum), nose, mouth, and genital and urinary tracts.

The different types of bacteria that make up your microbiome can have good, neutral, or negative impacts on your body. The microbes that have good effects are called beneficial bacteria, or probiotics. Probiotics are like invisible workers that positively influence countless aspects of your body, including energy, digestion, brain activity (behavior and emotions), drug metabolism, the release of essential vitamins, protection against infection, and more (Trinh et al. 2018).

Your microbiome gets its start before you are born. Researchers have found that fetuses swallow tiny amounts of maternal gut bacteria floating in the fetal amniotic fluid (i.e. the “waters” inside your amniotic sac). But the bulk of your microbiome is seeded at birth, when you are exposed for the first time to your birthing parent’s genital tract and/or skin (Gensollen et al. 2016).

In fact, one reason that newborns born by vaginal birth tend to have better health outcomes in infancy and childhood is because a microbiome seeded by a vaginal birth tends to have more beneficial bacteria at a young age (when your immune system is developing) than a microbiome seeded by a Cesarean birth (Milani et al. 2017).

Some parents who give birth by Cesarean may attempt to replicate the microbiome received after a vaginal birth by wiping their baby’s mouth, face, and skin with the birthing person’s vaginal fluids, in a practice known as “vaginal seeding.” This practice is controversial, and we will not cover it in this article, but you can learn more about it here.

Infant feeding methods also have a strong influence on the microbiome. Human milk contains antibodies, or immune properties, that fight bad bacteria and prevent it from finding a permanent home in the gut. Human milk also contains large amounts of oligosaccharides—complex sugars that feed beneficial bacteria. By boosting the beneficial bacteria, the oligosaccharides also positively influence a bodyfed newborn’s immune system and their ability to fight infections (Milani et al. 2017).

The microbiome is constantly changing in the first three years of life as you have close contact with new people, places, animals, foods, and fluids. After three years, your microbiome stabilizes and the bulk of it will remain the same through adulthood, except for when you experience infections and/or have close contact with different types of bacteria (Gensollen et al. 2016).

Some people who are not carriers of GBS bacteria in childhood may become GBS positive later in life through close or intimate contact with other people. However, Group B Strep is not considered a sexually transmitted infection for two main reasons: 1) it’s a naturally occurring bacteria present in many people who are not sexually active, 2) it rarely causes infection or problems unless you are a newborn, elderly, or immunosuppressed (Steer and Plumb, 2011).

Group B Strep can join your body’s microbiome if you have close contact with someone else who carries GBS… just like how other bacteria may be introduced into your microbiome through close contact with others. There is no evidence that Group B Strep is spread by water, food, or surfaces (CDC, “About Group B Strep,” 2022).

What causes early Group B Strep disease?

Right before a baby is born, the microbiome has not been seeded, and there is a window of opportunity for bad bacteria to gain a foothold in a fetus’s body (Gensollen et al. 2016).

Fortunately, there are multiple ways that fetuses are protected from infection during pregnancy. One major protective factor is the chorioamnion, also known as the fetal membranes, or the “bag” or “sac” surrounding the waters. The membranes are an important barrier that prevent bacteria from getting into the uterus and fetus (Parry & Strauss 1998).

However, once the membranes rupture (also known as your “waters breaking”), there is now a potential pathway for infection to occur. Group B Strep bacteria, if present, can travel from the vagina up into the amniotic fluid and uterus, in what is called an ascending infection or vertical transmission. The fetus may swallow some of the GBS bacteria into their lungs and possibly experience early GBS disease (Puopolo et al. 2019).

Early onset GBS disease is defined as detecting GBS in the blood, cerebrospinal fluid, or lungs, along with the infant showing signs of clinical disease such as sepsis (bloodstream infection), meningitis, or pneumonia, during the first 6 days of life (Seale et al. 2014).

When a baby has early GBS disease, symptoms appear at birth or shortly after birth (Puopolo et al. 2019), and almost all babies (95%) will have symptoms within 48 hours (Nandruri et al. 2019). In a study of 148,000 infants born between 2000 and 2008, almost all of the 94 infants who developed early GBS disease were diagnosed within one hour after birth—which is why researchers believe that GBS disease usually begins before birth (Tudela et al. 2012).

Newborns can also become carriers of GBS when it meets their skin, nose, and mouth as they travel down the birth canal during a vaginal birth. However, most of these infants stay healthy (CDC 2010).

How common is early GBS disease?

A meta-analysis combining data from many studies around the world estimated that in the year 2015, out of 140 million live births, about 205,000 infants had early GBS disease (Seale et al. 2017).

Researchers have studied GBS and found that if a birthing person who carries GBS is not treated with antibiotics during labor, the baby’s risk of becoming a carrier of GBS is approximately 50% and the risk of early GBS disease is 1 to 2% (Boyer & Gotoff 1985; CDC 2010; Feigin, Cherry et al. 2009). As noted earlier, being a carrier of GBS is not the same thing as having early GBS disease– most carrier infants stay healthy.

On the other hand, if the birthing person with GBS is treated with antibiotics during labor, the risk of their infant developing early GBS disease drops by 80%. So, for example, the risk could drop from 1% down to 0.2%. (Ohlsson 2013)

In the U.S., the Centers for Disease Control (CDC) runs the Active Bacterial Core surveillance (ABCs), a program that tracks potentially invasive bacteria in a sample of ten states, covering a population of 45 million people.

In the early 1990s, before the American College of Obstetricians and Gynecologists (ACOG) and the American Academy of Pediatrics (AAP) recommended universal screening for GBS and treating it with IV antibiotics during labor, there were 1.8 cases of early GBS disease per 1,000 live births (ACOG 2019).

After the newer recommendations went into action, rates of early GBS disease in the U.S. dropped drastically. In the most recent data set, the ABCs program found that there are only 0.2 cases of early GBS disease per 1,000 live births (CDC, 2020).

In England, where the other risk factor approach is used to lower the risk of early GBS disease, the 2020 rate of early GBS disease was 0.53 per 1,000 live births, which is more than double the rate in the U.S. (UK Health Security Agency, 2021).

What is the risk of death with infant GBS disease?

Researchers who study global rates of GBS disease reported that in 2015 there were 51,000 deaths of infants with early GBS in places with no access to health care facilities, 27,000 early GBS deaths in developing countries with some access to health facilities (a mix of areas with access to health care facilities and other areas without access), and 500 early GBS deaths in developed countries with reliable access to health facilities (Seale et al. 2017).

When stillbirths from GBS, early GBS disease, and late-onset GBS cases were combined, GBS was a leading cause of fetal/infant death around the world, causing more than 127,000 infant deaths and stillbirths each year. The burden of infant mortality from early GBS disease is born disparately by low-resource countries. The countries with the highest annual numbers of infant GBS disease deaths include India (13,000), Nigeria (8,000), Ethiopia (4,000), the Democratic Republic of the Congo (4,000), and Pakistan (3,000) (Seale et al. 2017).

In the year 2015, it was estimated that in-labor antibiotics prevented approximately 29,000 cases of early GBS disease and 3,000 early newborn deaths, mainly in high-income countries that have access to this approach. If in-labor antibiotics were available worldwide, it is estimated that another 83,000 cases of early GBS disease and 27,000 infant deaths could be prevented each year (Seale et al. 2017).

In the U.S., which is considered a high-resource country by researchers (despite the high number of perinatal care deserts), approximately 6.9% of full-term infants with early GBS disease will die from their infection. Death rates are higher (19.2%) in preterm infants with early GBS disease (Nanduri et al. 2019).

Although newborns in high-resource countries will most likely survive if they have early GBS disease, their illnesses usually require long, expensive stays in a NICU. During these NICU stays, infants receive invasive interventions to treat serious illness from GBS-caused pneumonia, sepsis, or meningitis.

In middle- to high-resource countries, up to 18% of infants who survive GBS meningitis end up with moderate to severe neurodevelopmental problems, including long-term cognitive, motor, vision, or hearing impairment. Rates of long-term neurological problems are likely much higher in low-resource parts of the world (Kohli-Lynch et al., 2017).

Very little is known about the long-term health risks of infants who have GBS with sepsis or pneumonia, but some may have long-term developmental problems as well (Kohli-Lynch et al., 2017).

Which newborns are more likely to get early GBS disease?

The main risk factor for early GBS disease is when the birthing person is a carrier of GBS.

However, there are some other factors that increase the likelihood that a baby may have early GBS disease:

  • Being born preterm* has been linked with a higher risk of the baby having early GBS disease, possibly because a GBS uterine infection can lead to preterm birth in the first place (Puopolo et al. 2019).
  • A long time period (18-24 hours or longer) between water breaking and giving birth* (Boyer & Gotoff 1985; Velaphi et al. 2003; Heath et al. 2009).
  • Birthing person has a high temperature during labor (> 38 C or 100.4 F)*  (Boyer & Gotoff 1985; Adair et al. 2003; Velaphi et al. 2003; Heath et al. 2009; Ying et al. 2019).
  • The waters broke before labor started, also known as premature rupture of membranes (Adair et al. 2003).
  • Infection of the membranes inside the uterus, also known as chorioamnionitis (Adair et al. 2003).
  • Having previously given birth to an infant with early GBS disease (a sign that there is heavy presence of GBS in the birthing person) (Puopolo et al. 2019).
  • Intrauterine monitoring during labor (Adair et al. 2003).
  • Membrane stripping or sweeping (Ying et al. 2019).
  • Identifying GBS in the urine at any point during pregnancy, which is a sign of a heavy presence of GBS (Carroll et al. 2016).
  • Giving birth for the first time (Carroll et al. 2016).

The items marked with an asterisk are some of the most important risk factors.

However, about 60% infants who develop early GBS disease have none of the risk factors on this bullet point list, except for the fact that the birthing person is a carrier of GBS (Schrag et al. 2002).

How are you tested for GBS? How accurate is the test?

In their most recent guidelines, ACOG recommends measuring GBS with a culture test at 36-37 weeks of pregnancy (not at 35 weeks, which used to be common).

To collect the sample for the culture test, a swab that resembles a long Q-tip will be used. The cotton tip should be inserted first into the vagina (not very far, only about 2 cm or ¾ inch), and then into the rectum (about 1 cm or ½ inch, through the anal sphincter). A speculum should not be used (ACOG 2019; ASM 2021).

Note: If GBS is identified in a urine culture during pregnancy, then the pregnant person is considered GBS positive and further culture screening does not need to take place (ACOG 2019).

After collecting the sample, the swab is placed in a special tube and sent to a laboratory for culture testing—where they see if the sample grows GBS bacteria. It takes about 48 hours to get the results back. People with severe allergies to penicillin should get additional results, called susceptibility testing, that describe whether the GBS they are carrying can be treated by alternative antibiotics (ACOG, 2019; ASM 2021).

When studying the accuracy of GBS tests, most researchers compare the culture results at 35-36 weeks (which used to be the standard practice, before it was changed to 36-37 weeks) to culture results from a sample collected during labor. A culture test during labor would be considered the “gold standard,” but because it takes 48 hours to get results back, this method is not used in practice.

Because people can test positive for GBS consistently or temporarily (results can change, depending on how much or how little GBS is present in your gut), the goal is to get results within at least 5 weeks of giving birth—which should be a reliable sign of whether GBS will be present in your birth canal during labor (ACOG, 2019).

In one high-quality study (Young et al. 2011), researchers did the culture test twice for each birthing person– once at 35-36 weeks and once more during labor. They compared the 35–36-week results to results from the culture taken during labor.

Of all the pregnant people who screened negative for GBS at 35-36 weeks, 91% were still GBS-negative when the culture was done during labor. The other 9% became GBS positive. These 9% were “missed” GBS cases, meaning that they were carriers of GBS, but most (41 out of 42) did not receive antibiotics.

Of all the pregnant people who screened positive for GBS at 35-36 weeks, 84% were still GBS positive when the culture was taken during labor. However, 16% of the GBS-positive people became GBS-negative by the time they went into labor. These 16% received unnecessary antibiotics.

What are antibiotics, and how do they work?

Antibiotics are medications that target and kill bacteria.

Most people today take the existence of antibiotics for granted. However, before clean water and antibiotics were widely available, bacterial infections caused massive amounts of human suffering (and continue to do so in low-resource areas).

For example, if you lived in the year 1900 in the U.S.:

  • The average life expectancy was 47 years old.
  • The leading causes of death were pneumonia, tuberculosis, and bacterial diarrhea, which (together with diptheria, another bacteria) caused one-third of all deaths, and 40% of these deaths were among children under the age of 5.
  • Most (90%) of children who caught meningitis died.
  • Strep throat could lead to scarlet fever, which could cause heart damage and heart failure.
  • Cesareans had a very high maternal mortality rate (sometimes as high as 80-90%), mainly because it was impossible to treat internal infections transmitted during the surgery.

Many people claim that antibiotics were “discovered” in the 1920s. However, that narrative is false. For millennia, humans have been identifying and using antimicrobial agents that occur naturally in nature. The first written description of antibiotics was in a papyrus from Egypt (dated 1500 BCE), where they described putting moldy bread on wounds. Researchers have found traces of tetracycline, an antibiotic, in ancient human skeletal remains in Sudan and Egypt. And antimicrobial agents are present in some herbs that have been used for thousands of years in Traditional Chinese Medicine. However, antibiotics were not mass produced and widely available until World War II (Hutching et al. 2019; Aminov 2010).

The story goes that in 1920, a British scientist named Alexander Fleming went away on vacation. When Fleming came back to his laboratory, he discovered he had accidentally left out a petri dish containing Staphylococcus aureus bacteria.

Something in the air—a Penicillium mold—had landed on the dish and emitted a substance that killed the bacteria everywhere it encountered it. Fleming named the substance penicillin, and over the next two decades, other researchers (including Ernst Boris Chain and Howard Florey) isolated and purified the compound—work for which the three of them won the Nobel Prize.

Eventually, penicillin was synthesized in factories and became a lifesaving medication during World War II. Infections and injuries that would have previously killed soldiers were now treatable.

Antibiotics also made their way into the delivery room. Fatal illnesses such as childbed fever, also known as puerperal sepsis (infection from group A streptococcus bacteria; transmitted from doctors’ hands to the vagina and uterus) began to become a tale of the past (although puerperal sepsis continues to kill women in childbirth in low-resource areas of the world).

But antibiotics are not perfect. Certain antibiotics are only effective at killing specific bacteria. Some antibiotics (such as penicillin) have a narrower range of action, and others (such as vancomycin) can kill a broader swath of bacteria (these are called broad-spectrum antibiotics).

Antibiotics have side effects—they can cause rashes, nausea, vomiting, diarrhea, and decreased appetite. Broad-spectrum antibiotics have the strongest (worst) side effects, because they kill off more of the good bacteria in your microbiome. This creates an opening for other microbes to grow and flourish, such as C. diff, a bacteria resistant to many antibiotics that can cause life-threatening diarrhea, or Candidiasis, a fungus that causes yeast infections.

Some people may develop an allergy after being exposed to an antibiotic. However, most adults (80-90%) who say they have a history of penicillin allergy can tolerate penicillin. This is because the allergy can disappear over time. In other situations, the previous “allergy” symptoms were related to something else (like the original infection). Still, life-threatening allergic reactions, called anaphylaxis, can happen in some people who are at high risk for a severe reaction (ACOG 2019).

Bacteria can also evolve to resist the effects of an antibiotic. This is called antibiotic resistance. The more often antibiotics are used, the more likely it is the targeted bacteria will evolve their DNA to become resistant… and antibiotic-resistant bacteria then spreads. Doctors and scientists around the world have labeled antibiotic resistance a global threat. Some have predicted that by the year 2050, without urgent discovery of new antibiotics, ten million people per year will die from antibiotic resistant infections (Hutchings et al. 2019).

If a strain of bacteria in your body has NOT evolved to be resistant to a specific antibiotic, then that is called antibiotic susceptibility. For example, Group B Strep is almost always susceptible to penicillin (CDC 2010; Melo et al. 2016; Nogacka et al. 2017).

What is the evidence on antibiotics during labor to prevent early GBS disease?

In 1973, a researcher proposed giving pregnant people penicillin to prevent early GBS disease in infants (Franciosi et al. 1973). So, researchers tried giving penicillin to pregnant people before labor, but this didn’t work. Although penicillin temporarily lowered GBS levels, by the time labor started the GBS levels were back up again, showing that GBS grows back quickly once the antibiotic is removed from your system (Gardner et al. 1979).

Next, researchers tried giving antibiotics during labor to those with GBS. In the late 1980’s, three separate groups of researchers in the U.S., Spain, and Finland randomly assigned pregnant people with GBS to either receive IV antibiotics during labor (penicillin or ampicillin) or no antibiotics (Boyer & Gotoff 1985; Tuppurainen and Hallman 1989; Matorras et al. 1991). Researchers combined the data from these 3 studies, with a total of 500 pregnant people, in a Cochrane review that was last revised in 2014.* The results showed that when participants with GBS had antibiotics during labor, the risk of their infants developing early GBS disease dropped by 83% (Ohlsson & Shah 2013).

*Studies with this design—randomly assigning people with GBS to antibiotics or no antibiotics—are highly unlikely to ever happen again. Since we have evidence that antibiotics during labor can prevent early GBS disease in newborns (including large observational studies detailed below), most researchers would consider it unethical to withhold antibiotics for the sake of research. So, these studies from 1985, 1989, and 1991 are probably the last randomized, controlled trials we will ever see on this topic.

As the Cochrane reviewers noted, there were quite a few limitations to these 3 studies. In their summary, the reviewers said “There is no valid information from these three small, old, and biased trials to inform clinical practice.” Health care institutions in some countries, such as the United Kingdom, hold the same viewpoint, and thus do not universally screen for GBS and treat with antibiotics during labor.

However, an alternative perspective would be that there is some valid information from these studies, along with some limitations to the evidence. Let’s break this down more.

How can we interpret the Cochrane review?

We cite Cochrane reviews often in our work here at Evidence Based Birth®. The Cochrane Collaboration is a highly respected, not-for-profit organization that conducts meta-analyses on different topics related to healthcare. A meta-analysis is a type of research study in which researchers pool statistics from previous studies into one larger study.

The Cochrane Pregnancy and Childbirth Group has a policy that they only do meta-analyses on randomized, controlled trials. So, the Cochrane review on GBS (published in 2009 and “updated” but essentially unchanged in 2014) only includes the three small randomized, controlled trials we mentioned above. They do not include other types of evidence, such as evidence from large observational studies where some people received antibiotics and others did not.

The researchers who wrote the Cochrane review on Group B Strep came to strong conclusions against the use of antibiotics for Group B Strep. After reviewing the three existing randomized, controlled trials on GBS, they stated, “It is remarkable that in North America the commonly implemented practice of intrapartum antibiotic prophylaxis to GBS colonized women has been so poorly studied.”

It is true that these three studies had some major limitations. In fact, most studies published before 1996 suffered from poor quality reports of their findings. In the mid-1990’s, researchers were very concerned about the widespread quality problem with clinical trial reports. So, in 1996, researchers from Canada and the U.S. came together and published the CONSORT guidelines for clinical trials.

CONSORT stands for the Consolidated Standards of Reporting Trials, and it is basically an evidence-based checklist of items that researchers must disclose in their article before they can report the results of their studies in most medical journals. Publishing of the CONSORT guidelines forced researchers to be transparent about their methods, and this greatly improved our ability to evaluate the quality of a clinical trial. The CONSORT guidelines were updated in 2001 and again in 2010. The 2010 version overrules the previous versions, but the biggest changes to clinical research happened after the first CONSORT guidelines were released in 1996.

The three studies that the Cochrane reviewers critique as being “invalid” were published in 1986, 1989, and 1990, before the CONSORT guidelines were developed. So, this partially explains why the written reports of these three studies are not up to today’s standards.

Keeping in mind the information about the Cochrane reviews and the history of clinical trials, Table 1 shares two perspectives on the Cochrane review findings on giving antibiotics during labor for GBS.

Table 1: Cochrane Perspective vs. Alternative Perspective


In summary, although these studies had limitations (not uncommon for research published before 1996), there is also some valid information that we can use.

Although it would be best if we had modern, larger, randomized, controlled trials on antibiotics for Group B Strep, such trials will not be available given that antibiotics are already in routine use, and it could be considered unethical to withhold antibiotics. We also have newer evidence from large observational studies that we can use to look at the potential benefits and risks of antibiotics for GBS.

What research is available from observational studies?

In 1996, after the original randomized trials were published on GBS, the Centers for Disease Control (CDC) in the U.S. initially recommended two ways to prevent early GBS disease: 1) the universal screening approach, or 2) the other risk factor approach. Because health care providers were choosing to use one or the other approach, scientists in the U.S. had a unique opportunity to study the effects of the two different methods.

In a study published in the New England Journal of Medicine (Schrag et al. 2002), researchers used CDC lab results and chart reviews to look at 629,912 live births that took place in the U.S. between the years 1998-1999.The researchers randomly selected 5,144 of these births to study, in addition to all 314 infants who were born with early GBS disease. They used hospital records to label people as receiving either the universal screening approach (52%) or the other risk factor approach (48%).

The results? The universal screening approach worked better than the other risk factor approach at preventing early GBS disease. Overall, there were 0.5 infants born with GBS per every 1,000 births. People in both groups received antibiotics about 33% of the time. But people whose care providers used the universal screening approach had a 54% reduction in the risk of early GBS disease compared to people whose care providers used the other risk factor approach.

The findings from this landmark study convinced the CDC to revise their guidelines in 2002 and only recommend the universal screening approach. 

In 2002-2003, researchers looked at 819,528 births in the U.S. to see whether the new guidelines had been put into practice. Like the previous study, the researchers picked a random sample of birthing people and infants to analyze, along with 254 infants who had early GBS disease. Between 1999 and 2002, use of the universal screening approach rose from about 50% to 85%, and use of antibiotics during labor rose from 27% to 32%.

This time around, there were 0.32 infants born with early GBS per every 1,000 birthing people (down from 0.5 cases per 1,000 only four years earlier). When researchers looked at term infants with early GBS disease, only 18% were born to pregnant people who were not screened. Most of the cases of early GBS disease in term infants (61%) happened in pregnant people who had been screened but tested negative for GBS (in other words, the test was a false negative). The researchers concluded that universal screening and antibiotic use cannot prevent all cases of early GBS disease. To lower GBS disease rates even more, a community would need easy access to vaccines against GBS (Van Dyke et al. 2009).

In 2019, when researchers (Nanduri et al. 2019) published another analysis of early GBS disease in the U.S. and the effects of universal screening as prevention, they found that GBS disease continued to drop further (to 0.23 per 1,000 live births). When they analyzed the 1,277 infants who had early GBS disease over a 10-year period (2006 to 2015) in ten states, they found:

  • In 48% cases there was no medical reason for the providers to give antibiotics:
    • In most (83%) of these cases the screening test was negative for GBS (a false negative result).
    • In the remaining cases, the GBS screening tests results were unknown.
  • In 21.8% of cases there was an indication for antibiotics, but the antibiotics were not provided. For example, the birthing person had a history of GBS in the urine or had pre-term labor.
  • In 29.9% of cases the birthing person had received a recommended antibiotic, but the infant still developed early GBS disease:
    • Most (60.9%) of these birthing people did not receive at least 4 hours of antibiotics.
    • In some of the cases (39.1%) the appropriate antibiotic was started 4 or more hours before the birth.

In a meta-analysis (Russell et al. 2017) that combined data from 30 studies around the world with more than 20,000 pregnant people, researchers compared rates of early GBS disease in locations where: 1) there is no policy/method being used for preventing GBS disease, 2) there is a policy of universal screening for GBS and treatment with antibiotics during labor (with varying levels of implementation).

They found that in places where there is no policy for GBS disease prevention (common in many low-resource areas), the risk of early GBS disease if the birthing person is a GBS carrier is 1.1%. In settings where there was a policy of screening for GBS (and if antibiotics were given correctly 75% of the time), the risk of infants experiencing early GBS disease was 0.3%. These percentages line up with what was reported in the original randomized trials on Group B Strep prevention.

What is the best time to receive antibiotics for GBS?

As we mentioned earlier, Nanduri et al. 2019 found that about 30% of cases of early GBS disease occurred even though the birthing person received antibiotics. But in most of those cases, the antibiotics were not given at least 4 hours before the baby’s birth. Other researchers had similar findings—that 4 hours or more of antibiotics is the most effective way to prevent early GBS disease.

Fairlie et al. found that when penicillin or ampicillin is given to someone who is GBS positive more than 4 hours before birth, it is effective 89% of the time. In contrast, giving antibiotics 2-4 hours before birth was effective 38% of the time. Antibiotics given less than 2 hours before birth were also not as effective as antibiotics given more than 4 hours before the birth. When clindamycin (another antibiotic) was used in place of penicillin, it worked very poorly (it was only 22% effective). This is not surprising, because many strains of GBS are resistant to clindamycin.

In another study, researchers reviewed the medical records of 4,756 birthing people who received antibiotics during labor for GBS– 1,149 received antibiotics for less than 4 hours, and 3,633 receiving antibiotics for 4 or more hours. There was a higher rate of newborn sepsis in cases where less than 4 hours of antibiotics had been delivered, as compared to receiving 4 hours or more of antibiotics (1.4% versus 0.4%.) (Turrentine et al., 2013).

In Uruguay, researchers followed 60 carriers of GBS at term who came to the hospital in early labor. They swabbed each person for GBS before antibiotics were started, and then again 2 and 4 hours after the first dose of penicillin was given. The researchers found that 72% of the participants were GBS positive before antibiotics were started, 47% were positive 2 hours after the first dose, and only 12% were positive for GBS 4 hours after the first dose. In all 60 newborns, the cord blood and amniotic fluid reached therapeutic levels of penicillin, even though 28% of the women gave birth before 4 hours. The maximum effect of the antibiotics was reached at 4 hours, just before the next dose was due (Scasso et al. 2015).

Because it takes time for antibiotics to pass to the fetus at therapeutic levels and kill off GBS bacteria (Scasso et al. 2015), it makes sense that giving the first dose of antibiotics too late (for example, a few minutes before the birth) might not help to stop a GBS infection from growing in the fetus. Based on research evidence, ACOG recommends that antibiotics for GBS should be started at least 4 hours before birth. However, they also state that medically necessary interventions shouldn’t be purposefully delayed just to get the 4 hours of antibiotics in before the birth of the baby. And although 4+ hours of antibiotics is ideal, 2 hours of antibiotics can still provide partial protection (ACOG 2019).

How will antibiotics during labor affect a newborn’s microbiome?

At the beginning of this article, we discussed the importance of the microbiome, and how it is seeded at birth. Unfortunately, antibiotics don’t just affect the bad bacteria that they are targeting—they also impact other bacteria, including good bacteria, or probiotics.

When this Evidence Based Birth® article was first published in early 2014, there was no research on the microbiome effects of in-labor antibiotics for GBS. Since that time, researchers have published two meta-analyses and at least nine individual studies on the microbiota consequences of IV antibiotics during labor for Group B Strep.

We reviewed nine studies (Table 2) in which researchers enrolled infants who were exposed to antibiotics during labor (typically for GBS) as well as infants who were not exposed to IV antibiotics during labor. Most researchers studied the infant gut microbiome by collecting and analyzing bacteria in stool samples at different time points, ranging from two days of life to one year of age.

Overall, researchers found that receiving IV antibiotics during labor negatively impacts the infant’s microbiome, at least temporarily. Eight of nine studies found that IV antibiotics during labor had at least a short-term effect on reducing beneficial bacteria and/or increasing levels of non-beneficial bacteria. Of the five studies that followed the infant microbiome over time, two found that the infant’s microbiome had either recovered or mostly recovered by 4 to 8 weeks, while three other studies found important differences that persisted up to three months or a year later in some infants.

Perhaps the most important study on this topic, and the only study to follow infants for a year, was conducted by researchers in Canada (Azad et al. 2016). Infants could be included in the microbiome study if they had stool samples collected at three months and one year, and if they had complete information about antibiotic exposure during labor and infancy.

The 198 infants in this study were separated into four groups:

  • No antibiotic exposure during labor with vaginal birth (57%).
  • Antibiotic exposure with vaginal birth (21%).
  • Antibiotic exposure with elective Cesarean (9%).
  • Antibiotic exposure with unplanned Cesarean (13%).

Cefazolin was the antibiotic that was typically used during Cesareans, and penicillin was the antibiotic of choice for vaginal births. Researchers also measured the presence and duration of exclusive breastfeeding/chestfeeding.

The results showed that the infant microbiome was influenced by antibiotic exposure during labor, birth route (Cesarean or vaginal birth), and breastfeeding/chestfeeding. At three months, infants exposed to antibiotics during labor or birth had a decreased level of Bacteroidetes (a beneficial bacteria), as well as a decrease in the “richness” of their microbiome, regardless of whether they were exclusively breastfed/chestfed or not. The most severe deficiencies happened among infants born by Cesarean. Infants born by Cesarean also had higher levels of Clostridium, Enterococcus, and Streptococcus—potentially harmful bacteria.

At one year of age, most of these differences were gone, showing that antibiotics only have a short-term effect on the infant microbiome. However, some negative effects on the microbiome persisted in infants born by unplanned Cesarean who had not been breastfed/chestfed for at least 3 months. And other researchers caution that temporary changes in the microbiome during that first year could still have a negative long-term impact on the developing immune system (Zimmerman & Curtis 2019).

The changes in the microbiome seen in all these studies are consistent with what one would expect after administering IV antibiotics, like penicillin and ampicillin, that mainly act against gram-positive bacteria. Killing off gram-positive bacteria (like Group B Strep) can lead to an over-abundance of gram-negative bacteria. Also, some beneficial bacteria, like Bacteroidetes, are sensitive to penicillin and ampicillin, meaning that they are also killed off by the antibiotic.

In summary, it does appear that IV antibiotics during childbirth have a short-term negative effect on the infant’s microbiome, but that this negative effect can be lessened by vaginal birth and breastfeeding/chestfeeding. Research is still needed to determine if there are any long-term immune effects associated with the temporary reduction in beneficial bacteria.

Parents who receive antibiotics and have a Cesarean, or those who choose not to breastfeed/chestfeed may need support from care providers in using other methods (such as donor human milk, probiotic supplementation, or formula with probiotics) to help their baby recover from any negative microbiome impact.


What are the potential benefits and harms of the universal screen and treat approach?

Potential Benefits:

  • In clinical trials and in observational studies, giving antibiotics (penicillin or ampicillin) to GBS carriers during labor decreases the risk of early GBS disease by 83% (Ohlsson 2013; Russell et al. 2017).
  • Penicillin and ampicillin rapidly cross the placenta into the fetal circulation (at non-toxic levels) and can prevent GBS from growing in the fetus in most cases (ACOG 2019; Barber et al. 2008).
  • In a large observational study comparing both approaches, the universal approach (screening and treating all GBS-positive patients with appropriate antibiotics during labor) was associated with lower rates of early GBS disease than giving antibiotics based on other risk factors (Van Dyke et al. 2009).
  • Most researchers have found that antibiotic resistance has not been a problem with penicillin, the drug most commonly used to prevent early GBS disease (CDC 2010; Melo et al. 2016; Nogacka et al. 2017).

Potential harms:

  • Although rare, severe allergic reactions in birthing people have been reported. In one study of more than 700,000 births in Texas, there were 19 cases of anaphylaxis– 5 due to penicillin and 6 due to cephalosporins, with zero deaths (Mulla et al. 2010). A meta-analysis of all studies published between 1985 to 2021 on severe anaphylaxis during pregnancy did not identify any maternal deaths due to an anaphylactic reaction to antibiotics (Simionescu et al. 2021).
  • IV antibiotics have a short-term negative effect on the infant’s microbiome. Whether or not this translates into a long-term effect on the developing immune system is unknown. The antibiotic’s negative effects on the microbiome can be lessened by breastfeeding/chestfeeding and vaginal birth.
  • Because antibiotics can kill off good bacteria as well as bad bacteria, antibiotics increase the risk of yeast infections, which thrive in the absence of good bacteria.
    • Yeast or fungal infections, known as Candidiasis, can harm the breastfeeding/chestfeeding relationship.
    • With a dual parent-infant yeast infection, the postpartum person may develop red/painful nipples, while their infant may experience thrush (inflamed, painful mouth).
    • In one study, 15% of people who received antibiotics in labor had postpartum yeast infections along with their baby, compared to 7% of those who did not have antibiotics (Dinsmoor et al. 2005).
  • Other potential harms have to do with side effects related to the antibiotic that is used (click on the link to see a comprehensive list of potential side effects for each antibiotic, but keep in mind that most of the serious risks are rare): penicillin, ampicillin, cefazolin, clindamycin, and vancomycin. See more information about clindamycin and vancomycin in the section below.
  • Antibiotics contribute to the medicalization of labor and birth because of the need for early hospital admission, IV insertion, and IV medications (RCOG 2017).

What are the best antibiotics for someone who is allergic to penicillin?

As mentioned earlier, most adults who say they have an allergy to penicillin are no longer allergic to penicillin. However, some people remain at high risk for severe allergic reactions. In their guidelines on GBS, ACOG (2019) provides a flow chart to help determine which antibiotic should be used for GBS in cases where there is a suspected penicillin allergy.

The strategy that ACOG recommends includes:

  • During a prenatal visit, the birthing person should describe any past allergic reaction to penicillin.
  • If their history indicates that they have a low risk of penicillin allergy, then they can receive cefazolin instead:
    • It used to be taught that 8-10% people with a penicillin allergy could be “cross-allergic” to cefazolin, but now we know that situation is less common (<1% to 4%). Most people who are allergic to penicillin can tolerate cefazolin.
    • Another option would be to undergo skin testing to determine if they have a true penicillin allergy.
  • If the history indicates that they are at high risk for a penicillin allergy:
    • The third trimester GBS culture should test for clindamycin susceptibility.
    • If the strain of GBS is susceptible to clindamycin, then clindamycin can be given.
    • If the strain of GBS is resistant to clindamycin, then vancomycin can be given.
    • Another option would be to undergo skin testing to determine if they have a true penicillin allergy.
  • People with an unknown risk of allergy to penicillin (for example, they aren’t sure if they are allergic or not) can have skin testing, take cefazolin, or receive clindamycin or vancomycin (depending on the GBS culture results).
  • Erythromycin should never be used for GBS. Most strains of GBS are resistant to erythromycin, and erythromycin does not cross the placenta.

Note: Ampicillin is a type of penicillin—so if you’re allergic to penicillin, you should not take ampicillin.

As you may have noticed from the ACOG guidelines, most people who have an allergy to penicillin can take cefazolin instead. One advantage to cefazolin is that (like penicillin) it crosses the placenta and reaches the fetal bloodstream. Also, it has a narrow range of action, is not as harmful to the microbiome as clindamycin and vancomycin, and can be given every 8 hours (instead of every 4 hours like penicillin).

If someone cannot take penicillin, ampicillin, or cefazolin, the two alternate options—clindamycin and vancomycin—have never been tested in clinical trials for the prevention of early GBS disease. Clindamycin faces high rates of drug resistance, barely reaches the fetal bloodstream, and should never be used unless the GBS strain is shown in culture testing to be susceptible to clindamycin.

Vancomycin, a broad-spectrum antibiotic, attacks many different types of bacteria. As a result, it has more frequent side effects, more serious side effects, and a larger impact on the microbiome. Also, overuse of vancomycin increases the risk that our communities will develop antibiotic resistance to vancomycin. Vancomycin’s serious potential side effects means that there must be safety protocols in place, such as giving it slowly (over 1-2 hours), checking kidney function (via a blood test before the drug is given), and using weight-based dosing.

Weight-based dosing with vancomycin means that the dose given is based on the birthing person’s weight:

  • When vancomycin is given without respect to the birthing person’s weight (about 1 gram given every 12 hours), then it reaches the fetal bloodstream only 9% of the time.
  • A lower weight-based dose (15 mg per kg given every 12 hours) reaches the fetal bloodstream 33% of the time.
  • A higher weight-based dose (20 mg per kg every 8 hours) reaches the fetal bloodstream 83% of the time.

If vancomycin is being given at a dose > 1 gram, it may need to be administered even more slowly than usual (over at least 2 hours). Premedication with an antihistamine such as Benadryl may help prevent flushing and other uncomfortable side effects due to the vancomycin. Also, the Association of Ontario Midwives states that, “Due to the controlled conditions under which vancomycin is administered, home birth is not a feasible option for GBS-positive individuals who choose IAP and whose only choice of antibiotic is vancomycin.”

Unfortunately, the American Academy of Pediatrics does not consider clindamycin or vancomycin to be “adequate” prevention of early GBS disease, which sometimes mean more newborn monitoring will be recommended. See the FAQ for more information about this dilemma.

Are there any other options?

Other Risk Factor Approach

As we’ve discussed earlier, one alternative to the universal approach is the “other risk factor approach.” This is when you receive antibiotics based on other risk factors such as having a fever, or your water being broken for more than 18 hours.

Researchers have found that the percentage of birthing people who receive antibiotics is roughly the same whether you choose the universal screening approach or the other risk factor approach—about 30%. However, as already mentioned, evidence from large observational studies shows that the universal screening approach is more effective than giving antibiotics based on other risk factors.

Some countries have changed their policy to include GBS testing in their other risk factor approach, even though we do not have research yet on a combination approach (Le Doare et al. 2017). For example, birthing people might be treated with antibiotics if they are GBS positive PLUS they have fever or prolonged rupture of membranes. But in most cases of early GBS disease, the only risk factor is that the birthing person is a carrier of GBS. So, it’s possible that the combination strategy (of requiring a positive GBS test plus one other risk factor) will not prevent most cases of early GBS disease in newborns.

Chlorhexidine (aka Hibiclens)

Chlorhexidine is a topical disinfectant that kills bacteria on contact. It binds easily to the skin and mucous membranes. In the vagina, the anti-GBS effects of chlorhexidine last from 3 to 6 hours. Chlorhexidine has been shown to be safe, is easy to administer, and is inexpensive (Goldenberg et al. 2006). Hibiclens is a brand formulation that includes chlorhexidine. Most of the research studies have used chlorhexidine; however, in the U.S., many providers can only access Hibiclens.

In a Cochrane review that was last updated in 2014 (Ohlsson et al. 2014), researchers combined results from 4 randomized, controlled trials that compared vaginal chlorhexidine vs. a placebo or no treatment on the health outcomes of 1,125 infants born to parents who were GBS positive. The evidence from these studies was judged to be of very low quality using the GRADE system of evaluating clinical trials. The researchers removed a fifth trial that had been included in previous versions of the Cochrane review, because it did not include people with known positive GBS status. They also corrected a data mistake from the previous version.

The Cochrane reviewers found that chlorhexidine does not reduce the infants’ risk of becoming a carrier of GBS. They also found no difference in early GBS disease rates among those who used the chlorhexidine and those who did not. There were no cases of infant deaths from GBS in either group. The chlorhexidine group had higher rates of stinging and irritation. The researchers called for a large clinical trial to test chlorhexidine for the prevention of early GBS disease.

Chlorhexidine could be beneficial for pregnant people living in low-resource areas where access to antibiotics is limited. In their review of the literature, Goldenberg et al. (2006) found two studies from low-resource countries (Egypt and Malawi) where researchers administered chlorhexidine in the vagina every 4 hours during labor and then wiped the newborn with chlorhexidine shortly after birth. This is a lower level of evidence than the studies listed above, because neither of these were randomized, controlled trials. Instead, the researchers followed hospitals over a period of months when: 1) they did not use chlorhexidine, 2) they used chlorhexidine, and 3) they stopped using chlorhexidine.

In both studies, researchers found that when chlorhexidine was used in the vagina and wiped all over the newborn, there were immediate drops in newborn hospital admissions, newborn sepsis admissions, and newborn deaths due to infections. Unfortunately, researchers did not specifically count the cases of GBS disease, just the overall number of babies who had admissions for sepsis.

So, is chlorhexidine effective? Randomized, controlled trials show that in high-resource countries, applying chlorhexidine topically during labor does not reduce GBS carrier rates or early GBS disease. However, evidence from low-resource countries shows that chlorhexidine vaginal wipes PLUS newborn wipes may lower newborn sepsis rates in general. And some researchers have proposed that in low-resource areas, applying chlorhexidine to the umbilical cord stump (after the baby is born) could prevent GBS-related blood infections after birth (Islam et al. 2016).

Chlorhexidine might be better than nothing, but it does not prevent the rise of GBS in the baby’s blood and amniotic fluid unless it is given before the birthing person’s water breaks and repeated before the effect wears off. In contrast, IV antibiotics quickly get into the fetal blood circulation to prevent or stop GBS disease from growing in the body.


Garlic has antibacterial properties, and some websites recommend putting garlic in the vagina to eliminate GBS before the GBS test. However, there are only two laboratory-based studies on this subject:

  • One group of researchers (Cutler et al. 2009) put garlic extract and GBS in a petri dish together. They found that the garlic was able to kill the GBS within about 3 hours.
  • Another group of researchers (Torres et al. 2021) tested compounds from garlic and found that 4 of the compounds inhibited the growth of GBS in a petri dish.

Although garlic may seem like a promising natural treatment, it’s important to understand that this treatment has never been tested in humans (a letter to the editor written by Cohain in 2009 is sometimes cited as a study, but it is not a research study).

If garlic has any anti-GBS effect, it’s likely only temporary. Back in the 1970s, when researchers tried giving penicillin during pregnancy (instead of during labor), they found it temporarily lowered levels of GBS, but levels almost always went back up by the time someone went into labor. So, by temporarily using garlic, this could help you get a negative test result, but the effect may wear off very quickly and the GBS could grow back.

There is no other info on the possible benefits and risks of short-term or long-term garlic use in the vagina. It’s possible that garlic insertion in the vagina could have unexpected effects like premature rupture of membranes or increase other bacteria– even GBS– due to destruction of good bacteria. Until we have more research on potential benefits and harms, there are a lot of unknowns related to this treatment.


Researchers have been working to develop a GBS vaccine for multiple reasons:

  • In-labor antibiotics do not prevent GBS infection 100% of the time.
  • In-labor antibiotics have side effects and can negatively impact the microbiome.
  • In-labor antibiotics do not prevent other GBS diseases, such as preterm labor, stillbirth, or late-onset GBS disease in newborns.
  • Pregnant people in low-resource parts of the world have difficulty accessing GBS testing and antibiotics during labor.

In an analysis published in 2017, Seale et al. predicted that, with a GBS vaccine that works 80% of the time and is administered to more than half of pregnant people in the world, we could prevent 23,000 stillbirths, 127,000 cases of early GBS disease in infants, and 37,000 infant deaths each year. They also calculated that vaccinating half of all pregnant people worldwide would be more effective at preventing disease and death than carrying out GBS screening and providing in-labor antibiotics to more than 50% of pregnant people.

There have been at least five pharmaceutical companies involved in developing GBS vaccines: Pfizer, GlaxoSmithKline (GSK) which acquired Novartis, Biovac, and Minervax. Most of the vaccines are in earlier phases including discovery, preclinical, and phase 1 testing. The Novartis/GSK GBS vaccine candidate has completed phase 2 safety testing in pregnant people.

In a 2020 study on the experimental Novartis/GSK vaccine for GBS (Swamy et al.), researchers carried out a randomized, placebo-controlled safety study in the U.S. They recruited healthy pregnant people between 24 and 34 weeks of pregnancy. All the participants received standard GBS testing and antibiotics during labor for GBS positive status—and 37% of the sample was positive for GBS.

Out of the 75 participants, 49 received one dose of the GBS vaccine and 26 received a placebo. The researchers followed the participants through the rest of pregnancy, birth, and up to 6 months postpartum. To measure the antibody response, they collected blood samples from the pregnant people on day 1 and 31 after vaccination, the day of giving birth, and postpartum days 42 and 90. They also collected infant cord blood samples at birth and infant blood samples on days 42 and 90.

The most common side effects were injection site pain (50% in vaccine group and 31% in the placebo group) and fatigue (38% of the vaccine group and 23% of the placebo group). There were zero vaccine-related serious adverse events in the study, and no differences in pregnancy/birth outcomes between the two groups. At six months of age, they performed developmental assessments on the infants and found no differences between groups with cognitive, communication, fine motor, and gross motor skills.

On day 31 after receiving the vaccine, the vaccine group had an immune response that was 13-23 times higher than in the placebo group. Antibodies transferred to the fetus through the placenta, and antibodies were detected in the infant’s blood from birth through 90 days of follow-up. The vaccinated group also had high levels of antibodies in human milk samples.

Currently, public health experts are optimistic about the possible positive impact that GBS vaccines could have around the world:

“An effective maternal GBS vaccine offers an all-encompassing approach to reducing GBS disease, and, as vaccine strategies can achieve high coverage in even the most challenging settings, it is likely to be a more equitable intervention than intrapartum antibiotic prophylaxis. Maternal GBS vaccination has the potential to reduce this disease burden worldwide, within the next generation and including the poorest families” (Seale et al. 2017).


In theory, taking probiotics in pregnancy should help lower the odds that you will test positive for GBS in the third trimester. This is because beneficial bacteria, such as Lactobacillus, make the vagina more acidic, which inhibits growth of GBS (Hanson et al. 2022).

In a meta-analysis published in 2022, Hanson et al. reviewed six studies (3 randomized trials, 2 observational studies, and 1 semi-randomized trial) that examined whether oral probiotics (primarily Lactobacillus) can lower rates of GBS in pregnancy.

The authors determined that probiotics had a moderate effect on reducing rates of testing positive for GBS in pregnancy. However, five of the six studies in this meta-analysis were at high risk for bias, meaning that we can’t have confidence in their results. For example, in one study, pregnant people were put into probiotic and control groups based on whether they were sick and wanted to take probiotics or not (leading to two very different groups), and no baseline measurement of GBS was collected. Another study also did not measure GBS before the probiotics were started, and it had to be shut down early because the probiotics expired.

Hanson et al. (2023) went on to carry out their own high-quality randomized trial, which was supposed to address some of the problems in the earlier research. Their study included these strategies:

  • They enrolled 109 healthy pregnant people, with the knowledge that at least 80 would need to complete the study to find a difference between probiotic and placebo.
  • The participants were randomly assigned to either receive 1) highly potent capsules with Lactobacillus and other probiotics (on the U.S. market as “Florajen Digestion”) or 2) placebo capsules. Both groups started the pills at 28 weeks of pregnancy and continued until labor.
  • Baseline swabs for GBS were collected at 28 weeks and again at 36 weeks.
  • The participants and study staff were not allowed to know which group each person was in in, and the probiotic and placebo pills were identical in appearance and taste—so it was a true double-blind study.
  • Adherence was measured with the use of special pill bottles that track how often the pill bottle was opened.
  • Probiotics were kept refrigerated and had frequency potency testing.
  • Participants received daily reminders to take their pills.
  • GI symptoms were measured by questionnaire at baseline, 28 weeks, and 36 weeks.
  • Yogurt ingestion, sexual activity, and vaginal cleansing practices were measured by questionnaire at baseline and 36 weeks.

In the end, 83 people completed the entire study protocol. The results showed that the probiotic group had fewer GI symptoms (per questionnaire) at 36 weeks compared to the placebo group. However, there was no difference in rates of testing positive for GBS at 36 weeks. The pill bottle caps showed that both groups took their assigned pills only about 51-60% of the time. There were no adverse health outcomes in either group.

In summary, the research on probiotics to lower the risk of testing positive for GBS is not very compelling. But no study has found bad side effects related to taking probiotics for GBS prevention, and at least three trials found that oral probiotics can lessen GI symptoms in pregnancy (Hanson et al. 2022; Hanson et al. 2023). So, although taking oral probiotics might not lower your chances of carrying GBS, it doesn’t seem to hurt, and it might help with other symptoms of pregnancy.

Human milk

Recently, researchers have begun to explore the possibility of applying human milk onto the vagina to suppress GBS. One of the main ingredients in human milk is oligosaccharides, also known as human milk oligosaccharides, or HMOs. This key component of human milk was first identified in the early 1930s, when scientists were trying to understand why survival rates were higher when infants were fed with human milk rather than formula based on cow’s milk (Bode 2012).

There are more than 100 types of HMOs found in human milk, and HMOs make up 8% to 20% of each ounce. HMOs are carbohydrates that are more complex than simple sugars, but less complex than starches. Scientists are still trying to understand the effects of HMOs, but so far HMOs have been shown to act as prebiotics (feed good bacteria in the gut), block bad bacteria from finding a home in the gut, positively impact the immune system, and provide nutrients for brain development, among other benefits (Bode 2012).

Farm animals (such as cows and goats) produce very few oligosaccharides, at levels that are 100 to 1,000 times less than human milk, and there is no other natural source where HMOs can be found. To mimic the effects of HMOs, non-HMOs are currently added to some infant formulas. Researchers have described short-term benefits of providing non-HMOs to formula-fed infants, but we still need research on long-term benefits and risks (Bode 2012).

Earlier in this article, we discussed how feeding an infant human milk can help lessen the negative effects of antibiotics on the newborn microbiome. However, recent research has been published that suggests topical use of HMOs could be a potential pathway for preventing or treating GBS during pregnancy.

In 2017, three different research teams found that HMOs could inhibit the growth of GBS in petri dishes (Lyon & Doran 2022). Then in 2022, researchers found that topical use of HMOs can inhibit GBS in the vaginas of laboratory mice. They also found that the topical HMO treatment did not disturb the rest of the vaginal microbiome in mice (Mejia et al. 2022). Human research has not yet been conducted on this potential use of HMOs.

In their commentary on Mejia et al.’s study, Lyon & Doran (2022) wrote, “It will be interesting to see how the field of anti-GBS HMO activity will evolve in the coming years and how in vivo models will continue to explore this promising therapeutic agent for improved maternal and neonatal health.”

Colloidal silver

A few websites mention colloidal silver as a remedy for preventing GBS infection. Although silver has anti-bacterial properties, no research studies have ever looked at the safety of colloidal silver in pregnancy.

In 1999, the Federal Drug Administration in the U.S. stated that colloidal silver is not safe or effective for any condition, and other researchers have reported serious toxicity and health risks.

Online misinformation about colloidal silver is common. For a peer-reviewed article for consumers with facts about colloidal silver, you can read this Healthline article.

Frequently Asked Questions:

Can I swab myself or does a health care provider have to collect the sample?

There have been at least two studies showing that culture results after self-swabbing for Group B Strep can be nearly as accurate as letting a health care worker collect the swab. As a result, guidelines from ACOG in the U.S. mention that pregnant people can collect their own specimens after receiving instruction.

In one study on this topic, Price et al. (2006) studied GBS testing among a diverse group of pregnant people in Canada. Most of the people they approached (86%) agreed to participate, showing that collecting a self-swab of the vagina and rectum (through the anal sphincter) was acceptable. The participants were given written instructions and a diagram showing how the swab was done. All the participants did the self-swab and had a swab collected by health care workers—but they were randomly assigned as to which swab was collected first. In the end, the culture test from swabs collected by health care workers was slightly more sensitive (96.9% vs. 87.5%), but the results were acceptable in both groups.

A few years later, Arya et al. (2008) carried out a similar study in Ireland. Everyone self-collected a vaginal-rectal swab and had a health care worker collect a vaginal-rectal swab (but they were not randomly assigned as to which would be collected first). The sensitivity was acceptable for both swabs, but it was higher for the clinician-collected swab (94.3% vs. 84.3%). Only 29% of participants preferred the self-collection, while 43% preferred the health care worker collection, and 28% had no preference.

Next, researchers from Hong Kong (Seto et al. 2019) found that pregnant people there may be hesitant to collect their own sample, leading to less accurate results. In this study, pregnant participants were asked to self-swab and be swabbed by health care workers—they were randomized as to which swab happened first. About two-thirds of those who were approached agreed to be in the study. Everyone watched a video on how to collect the sample—instructions included inserting a cotton tip swab 2 cm into the vagina and another swab 1 cm through the anal sphincter. The results showed 97.6% sensitivity for the swabs collected by health care workers, but only 61.4% sensitivity for the self-collected swabs. The researchers concluded that cultural factors may make some groups less inclined to perform an accurate self-screening test.

To sum up the research on this topic, self-swabbing can be a good option for people who prefer to collect the sample themselves. But other people may prefer (and be more comfortable with) a health care worker collecting the sample for them.

Is there a faster test that can be used in labor?

Processing a GBS culture takes at least 1 to 2 days, which means the sample must be collected before labor begins—this is why testing is performed at 36-37 weeks of pregnancy.

Unfortunately, because GBS levels can increase (be undetectable at first, but then increase and populate the vagina/rectum) or decrease (be detectable at first, but then retreat farther back into the intestines and no longer populate the vagina/rectum), this means someone who tests positive at 36 weeks might not be positive during labor at around 40 weeks, and vice versa.

If a faster, reliable test was available, then testing could be done during labor— thus identifying and treating only those whose birth canal is populated with GBS while giving birth.

In theory, this approach would result in fewer missed cases of GBS (for those who were negative at 36-37 weeks, but positive during labor), as well as less use of unnecessary antibiotics (for those who were positive at 36-37 weeks, but negative during labor).

When we first published this Evidence Based Birth® Signature Article in 2014, it seemed like rapid GBS tests might be coming soon. However, subsequent research results revealed that rapid GBS tests are unlikely to be common for the following reasons:

  • When clinicians use rapid tests in real life situations (as opposed to in a clinical trial, where circumstances are under strict control), rapid GBS tests are not as sensitive/accurate as culture tests (Mueller et al. 2014).
  • Even the most “rapid” rapid tests still require 1.5 to 2 hours to process, which is not helpful if labor is moving quickly (ASM 2021; Mueller et al. 2014).
  • Research is conflicting on whether rapid tests lower the rate of unnecessary antibiotics (Daniels et al. 2022; Poncelet-Jasserand et al. 2013).
  • Rapid tests for GBS cannot screen for antibiotic susceptibility in people with allergies to penicillin (ASM 2021).
  • Relying on a rapid test strategy requires a laboratory that is open and running 24 hours/day, which is not available in some hospitals (ACOG 2019).

However, rapid tests might be an option with preterm labor, or for situations where someone wasn’t tested or doesn’t have culture results back yet. For this reason, some hospitals keep a supply of rapid tests for GBS (ASM 2021).

If I have antibiotics, does this mean I will be continuously hooked up to an IV?

Not necessarily. If you use the antibiotics, you will have an IV placed, but it only takes a limited time for the antibiotics to run in. Each antibiotic is slightly different, but penicillin and ampicillin are usually infused over 15 minutes and given every 4 hours until birth, which for many people is only once or twice. For people with penicillin allergies, cefazolin is usually infused over 30 minutes and is given every 8 hours, clindamycin is infused over 10-60 minutes and given every 8 hours, and vancomycin is infused over at least 1 to 2 hours and given every 8 or 12 hours depending on the dose.

Some people may wonder why the doses need to be repeated. Evidence shows that as long as someone is in labor, the doses are repeated because the GBS-suppressing effect can wear off over time (ACOG 2019).

When the IV is running, it should not limit positioning, walking, or even laboring in water. For the hours in between, you can ask for the IV to be “hep-locked” or “saline-locked” and detached, so that you are free from the IV pole.

For more information about saline locks, please read our EBB article about saline locks during labor here.

Can infants acquire a GBS infection from staff handling the newborn?

Researchers are quite certain that infants usually catch early GBS disease before they are born—likely from GBS in the amniotic fluid. As mentioned earlier, almost all infants with early GBS infection show symptoms within an hour after birth.

Late-onset GBS disease usually starts between the age of 7 days and 3 months old. Treatment with antibiotics during labor does not prevent late-onset GBS disease in infants (ACOG 2019). Infants can catch late-onset GBS disease from the hospital (nursery, hands of hospital staff and family members) or the community. This is one reason handwashing is so important during the newborn period (Kliegman et al. 2011).

My care provider says I need to be tested for GBS twice—once in early pregnancy and once more at 36 weeks. Is this evidence based?

We have heard of this practice happening in the U.S., but we have not seen anything published about it in either the clinical guidelines or in research. Perhaps these care providers are testing their clients early to get an idea as to whether you are at higher risk for pre-term birth. However, GBS is the cause of only 1-2% of pre-term births. 

And any preventive treatment of GBS during pregnancy is temporary, since the GBS almost always grows back in the intestines once the antibiotics are stopped.

However, if you have a UTI, and the urine culture grows GBS bacteria, then you do need to be treated with antibiotics to eliminate the GBS from your urinary tract.

If I am GBS positive, and I don’t get the IV antibiotics for some reason, what kind of tests will my baby need to have?

If a baby is born at 35 weeks or later, guidelines from the AAP (Puopolo et al. 2019) describe three different approaches that health care workers might use:

  1. Risk assessment by categories: There are different versions of this approach used around the world—see the AAP guidelines for a sample flowchart. If the birthing person carries GBS and received penicillin, ampicillin, or cefazolin 4 or more hours before birth, then the infant is labeled as having “adequate” early GBS disease prevention. On the other hand, if the birthing person carries GBS, and they received any other antibiotic, or antibiotics were given less than 4 hours before birth, then the infant would be considered as having “inadequate” prevention, and observation, blood cultures, or antibiotics may be prescribed, depending on the category that the infant falls under. A major drawback to this approach is that it results in interventions with many low-risk infants.
  2. Neonatal Early-Onset Sepsis Calculator: Information about the birth, GBS status, and infant’s health condition are entered into an evidence-based calculator developed through large, high-quality research studies. The calculator suggests different courses of action based on each infant’s individual score. The calculator is free and publicly accessible here: One benefit of this calculator is that it determines risk based on all kinds of infections, not just GBS.
  3. Risk assessment based on newborn’s health condition: Infants who appear well at birth do not receive any special tests or interventions. If the infant is well, but there was inadequate prevention of GBS or the birthing person had a fever during labor, then the infant should be checked frequently over the next 36-48 hours. Infants that appear sick at birth or develop signs of sickness over the first 48 hours after birth should have blood cultures drawn and be treated right away with antibiotics.

The AAP states that preterm infants born before 35 weeks 0 days gestation are at high risk for early infection from all causes, and health care providers should base their management on the circumstances of the preterm birth (for example, management of the infant would differ if the preterm birth was caused by a pregnant person’s infection vs. preeclampsia).

What are the signs of GBS disease in newborns?

It’s important that parents educate themselves about what normal newborn behavior and appearance, as well as the warning signs of illness. The Association of Ontario Midwives has an excellent handout that includes all this information in a printer-friendly format:

Contact your provider if your newborn infant has signs of early GBS disease. The American Academy of Pediatrics (Puopolo et al. 2019) and Association of Ontario Midwives list the following symptoms:

  • Fast heart rate (for example, more than 160 beats per minute).
  • Fast breathing (for example, more than 60 breaths every minute, with the fast breathing lasting more than 10 minutes)
  • Lethargy (unable to wake baby to feed)
  • Limp and not interacting with caregiver when awake
  • Fever of 38 degrees C (100.4 F) or higher
  • Poor feeding
  • Irritability
  • High-pitched crying, or crying all the time
  • Grunting or nasal flaring
  • Baby’s skin pulls in sharply around ribs or base of throat when breathing in
  • Repeated, projectile vomiting
  • Temperature instability
  • Bulging soft spots on the head
  • Brick dust color in baby’s diaper more than 3 days after birth
  • No wet diaper in 24 hours
  • Any other worries about the baby’s health

The Association of Ontario Midwives recommends calling 911 and your provider if your baby’s skin color changes to blue, grey, or pale (although temporary blue hands and/or feet can be normal in the first few days of life) or their breathing stops for more than 10 seconds.

Does GBS affect different communities differently?

In the U.S., researchers have reported disparities with African American birthing people having higher rates of GBS positive tests, and higher rates of GBS disease in newborns (Nanduri et al. 2019). The 2022 Clinical Practice Guidelines from the Association of Ontario Midwives report that Black pregnant people may be more likely to convert from GBS negative to GBS positive due to systematic health disparities or inequitable delivery of antibiotics, but overall, these disparities are not widely researched nor well understood.

The American College of Obstetrics and Gynecology lists “maternal black race” among their list of risk factors for infants developing GBS disease. However, the Human Genome Project confirmed in 2003 that humans are 99.9% genetically identical and there are often more biological differences found within a single race, than found between people of different races (ACOG 2019, Duello et al. 2021). Geneticists have described race as a social construct, not a scientific one, and it may be challenging to determine societal expectations of “race” means for patients with any historically mixed ancestry. Experts in racial health disparities such as Dr. Joia Crear-Perry, OB-GYN, and Founder of the National Birth Equity Collaborative often clarify with the explanation:

“Race isn’t the risk factor – racism is.”

Research shows that as result of racism (including racial weathering and chronic stress), Black, Brown, and Indigenous communities experience higher rates of hypertension and labor complications, preterm labor, Cesarean rates, low birth weight babies, and lower rates of breastfeeding/chestfeeding (Geronimus et al. 2006; Larrabee Sonderlund et al. 2021). More specific and more measurable than “black race,” some of these factors put infants at higher risk of developing GBS disease if exposed to GBS. Racial weathering also puts birthing people at risk of early cellular aging and a weakened immune system, making them vulnerable to allergies and imbalance in the microbiome gut bacteria (Carter et al. 2019). This puts them at risk of both becoming GBS positive and having a possible sensitivity or reaction to the antibiotics used to treat GBS.

Communities that experience discrimination in health care – for any reason – may also face real-time challenges from certain providers (whether the provider is aware or not), such as delays in care, less monitoring or less attentive monitoring, microaggressions, false beliefs about the patient, or unequal delivery of procedures and tests (Miller and Peck 2019, Hoffman et al. 2016, Smedley et al. 2003). Structural inequities may show up in low-resource areas where health centers face shortages of IV antibiotics, chronic understaffing and overworked providers, poor basic infrastructure or lack of access to laboratories for culture testing – all systemic factors that interrupt patients’ access to care.

For any birthing person with multiple layers of risk factors, prevention and extra wrap-around care and attentiveness can be lifesaving. Evidence-based preventive health options such as probiotic supplementation can be discussed early on with birthing people and families of color. Extended families, doulas, and community advocates can be trained on the signs and symptoms of GBS disease and sepsis (infection) in newborns and how to use the Neonatal Early-Onset Sepsis Calculator determine the likelihood of illness. This can be a helpful resource to support their case, especially if they feel a provider is not listening or acting urgently enough to treat their child. Parents and health care workers should also be aware that low oxygen can look different on darker skin (more grayish or whitish rather than blue), and that pulse oximeters are more likely to have inaccurate results in infants with darker skin tones (Vesoulis et al. 2022).

What do national organizations have to say?

In the United States:

In 2018, the U.S. Centers for Disease Control and Prevention transferred stewardship of GBS prevention guidelines to the American Congress of Obstetricians and Gynecologists (ACOG), the American Academy of Pediatrics (AAP), and the American Society of Microbiology.

The 2019 ACOG recommendations (reaffirmed in 2022 and endorsed by the AAP) include:

  • All pregnant people should be tested for GBS between 36 weeks 0 days and 37 weeks 6 days.
  • Everyone who is positive for GBS should have appropriate IV antibiotics during birth unless they are giving birth by pre-labor Cesarean and the membranes have not yet ruptured.
    • The antibiotic of choice is IV penicillin, and IV ampicillin is an appropriate alternative.
    • For people with a reported allergy to penicillin, if they are at low risk for an anaphylactic allergic reaction, then IV cefazolin is recommended.
    • For people with an allergy to penicillin who are at high risk for anaphylaxis, then clindamycin should only be used if laboratory testing shows that their GBS strain is susceptible to clindamycin.
    • Testing for a penicillin allergy is also appropriate for people who have history of penicillin allergy, as many people with childhood reactions will no longer be sensitive/allergic to penicillin later in life.
    • If someone has a high-risk allergy to penicillin, and their GBS strain is resistant to clindamycin, then IV vancomycin is the only other alternative—strict weight-based dosing and safe medication delivery measures (detailed inside the guidelines) should be followed.
  • Health care staff should not delay or withhold necessary medical interventions (rupture of membranes, synthetic oxytocin, membrane sweeping, Foley cervical ripening, vaginal exams, intrauterine monitoring, or water immersion during labor) just because someone is GBS positive and/or the health care worker wants to give the recommended 4 hours of IV antibiotics before the delivery.

The 2019 AAP guidelines of how to manage infants at risk for GBS disease were mentioned in the section above and can be found here.

Detailed laboratory testing guidelines can be accessed from the American Society for Microbiology here:

  • They recommend that a single swab should be used to obtain a specimen first from the vagina (inserting it 2 cm), then from the rectum (inserting it 1 cm through the anal sphincter), without using a speculum.
  • Antibiotic susceptibility testing should be carried out on samples from patients with severe penicillin allergies.

In the United Kingdom:

  • The United Kingdom National Screening Committee states that pregnant people in the U.K. should not be screened for GBS. Screening for GBS in pregnancy is not covered by the National Health Service (NHS). However, some pregnant people choose to be screened for GBS outside of the NHS.
  • In general, practitioners in the U.K. follow the other risk factor approach. This includes giving antibiotics during labor to anyone with fever, prolonged rupture of membranes >18 hours, GBS in urine at any time during pregnancy, preterm labor, or a prior infant with GBS. As a result, some people who would have tested negative for GBS receive antibiotics directed at GBS. In the U.K., the rate of early GBS infections is 0.53 per 1,000 births, which is higher than the rate of 0.2 per 1,000 births in the U.S.
  • The Royal College of Obstetricians does not recommend routine screening for GBS during pregnancy. However, they do state that in-labor antibiotics could be considered if the birthing person seeks GBS testing outside the NHS and tests positive, or if the birthing person has any of the other risk factors listed above.
  • There is controversy in the U.K. over the lack of access to GBS testing within the National Health Service. Group B Strep Support is an independent charity that advocates for families to have access to GBS screening in the U.K. They offer a support teleline, leaflets and publications, a private Facebook community, and links to order GBS tests online.

In Canada:

  • The Society of Obstetricians and Gynaecologists of Canada (SOGC) recommends offering GBS screening to all pregnant people at 35 to 37 weeks and providing those who test positive (as well as anyone with an infant previously infected or a positive urine culture) with the appropriate antibiotics during labor.
  • The Association of Ontario Midwives recommends GBS screening and has many resources including thorough evidence-based practice guidelines, a GBS app for midwives, sample protocols, and handouts for parents on GBS in pregnancy and normal newborn behavior vs. signs of illness. The AOM’s 2022 practice recommendations include:
    • Offer all midwife clients screening for GBS at 35 to 37 weeks; clients may be offered instruction on how to do the swab themselves.
    • Discuss the benefits and risks of penicillin allergy testing with clients who have an uncertain allergy.
    • Discuss the benefits and risks of two approaches: 1) screening for GBS at 35-37 weeks and having antibiotics during labor or 2) screening for GBS at 35-37 weeks but only having antibiotics in labor if an additional risk factor arises.

What is the bottom line?

Group B Strep is a normal part of the microbiome bacteria for some pregnant people, but it can cause early GBS disease (a serious illness) in a minority of newborns, because of their immature immune system. Researchers have found that if a pregnant person who carries GBS is not treated with specific antibiotics during labor (penicillin, ampicillin, or cefazolin), then there is a 1 to 2% chance their infant will develop early GBS disease.

There has been a global debate as to whether we should focus on preventing early GBS disease vs. preventing the harms of antibiotics. In places where preventing more cases of early GBS disease is the priority, people are typically screened for GBS at 36 to 37 weeks of pregnancy and then, if positive, provided antibiotics during labor (“universal screening approach”). If appropriate antibiotics are given 4 hours or more before birth, then the risk of early GBS disease goes down to about 0.2%.

In places where preventing the harms of antibiotics is the priority, people are not screened for GBS during pregnancy and only given antibiotics during labor if other risk factors occur (“other risk factor approach”). Although antibiotic usage rates are similar with the universal screening approach or the other risk factor approach, rates of early GBS disease are higher with the other risk factor approach (0.5% vs. 0.2%). An excellent informed consent handout that describes the benefits and risks of both approaches is available at the Association of Ontario Midwives’ website here:

Although the universal approach is the most effective approach, it is not perfect, and it cannot eliminate all cases of early GBS disease. Also, the universal screening approach is only available where there is easy access to laboratory testing and intravenous antibiotics. In low-resource places where no policy at all is used to prevent GBS disease (neither the universal approach nor the other risk factor approach), GBS disparately causes serious newborn illness and death.

Eventually, the most effective way of preventing GBS disease in infants would be access to a safe and effective vaccine against GBS for pregnancy. One GBS vaccine has shown promising results in phase II testing. Other alternatives to antibiotics have either not been shown to be effective against GBS disease or have not been sufficiently studied. For example, research from randomized trials does not support the theory that topical chlorhexidine (sometimes called Hibiclens) prevents early GBS disease. Topical insertion of garlic into the vagina has also been mentioned as a possible treatment—but has not been studied in pregnant people. Early onset GBS disease usually begins when GBS gains access to the amniotic fluid and gets into the fetus’ lungs during labor, so it’s unlikely that topical applications can prevent cases of GBS disease in newborns.

Regardless of what method of GBS prevention is used, parents should educate themselves on the warning signs of newborn sickness in case they need to seek emergency medical care. The Association of Ontario Midwives has an excellent handout on normal newborn behavior that includes a list of warning signs to seek emergency medical attention. You can access their handout in different languages here:

Regardless of which strategy is used (universal screening vs. other risk factor approach), about one-third of people in high-resource countries are given IV antibiotics during birth. These antibiotics have the temporary side effect of negatively impacting the infant microbiome. If you receive IV antibiotics, there might be an increased need for probiotic supplementation for your infant after birth if you:

  • Cannot or choose not to feed infant with human milk.
  • Give birth by Cesarean.
  • Preterm birth or NICU admission.
  • Have a high risk of penicillin allergy and thus receive a broad-spectrum antibiotic (clindamycin or vancomycin) that has a stronger impact on the microbiome and is less likely to prevent GBS disease.

On the other hand, parents might have increased concern about GBS disease if they have:

  • Culture results that show they are a GBS carrier.
  • Preterm labor.
  • Prolonged rupture of membranes.
  • Fever during labor.
  • History of urinary tract infections with GBS or a prior infant with GBS disease.
  • Improper culture testing at 36-37 weeks (an inaccurate test is more likely to occur if the Q-tip did not enter through the anal sphincter).

We encourage parents who face decisions around GBS testing and antibiotics to have open conversations with their providers about the benefits and risks of their options. The truth is, there are trade-offs to every decision. With solid information and access to resources, we believe each family has the power to make the decision that is best for their unique situation.

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Table 2: Research on the Newborn Microbiome after IV Antibiotics for GBS.

Table 2: Research on the Newborn Microbiome after IV Antibiotics for GBS.

First Author (Year) Sample and Methods Results Notes

Jaureguy (2010)


Researchers collected stool samples from 50 newborns in France on day 3 after birth. Half of the infants were born to birthing people who had IV amoxicillin for GBS during labor, while the other half did not. Infants were matched for gestational age, vaginal or Cesarean birth, and breastfeeding or bottle feeding. 49 of the 50 infants tested positive for GBS bacteria by day 3 of life. The only infant who was not came from the antibiotics group. Bacterial presence was similar between groups, but there were fewer infants with C. difficile (a harmful bacteria) in the antibiotic group. They also found no difference in rates with antibiotic-resistant strains of bacteria. This was the first study that compared bacterial colonization among infants who were exposed to in-labor IV antibiotics for GBS, versus those who were not exposed to IV antibiotics and the birthing people were not GBS carriers.
Fouhy (2012)

Researchers examined the gut microbiota of 9 newborns who were given IV ampicillin and gentamicin within 48 hours of birth. They took stool samples 4 and 8 weeks after the infants completed their antibiotics and compared the results to 9 untreated infants.


Infants who received antibiotics shortly after birth had much lower levels of beneficial bacteria, including Bifidobacterium and Lactobacillus. These good bacteria were replaced by members of the Proteobacteria family. Overall, the infants who received antibiotics shortly after birth had less diversity of their gut microbiome 4 and 8 weeks after antibiotic treatment, even though some beneficial bacteria had recovered.

The information from this study may or may not apply to birthing people who receive IV antibiotics during labor for GBS, because these infants received antibiotics after birth. The most common antibiotic given for GBS during labor is penicillin, with cefazolin, clindamycin, and vancomycin being given less frequently.


Keski-Nisula et al. 2013


Researchers collected vaginal swabs from 50 pregnant people approximately 21 days before birth. They also swabbed the baby’s mouth immediately after birth. They only included parent-infant pairs in the final data analysis if the birthing person had Lactobacillus present in the vagina prenatally, and they excluded babies born by elective Cesarean when the membranes were intact. 17 people received antibiotics during labor—indications for the antibiotics included GBS, suspected chorioamnionitis, and Cesarean. 45 of the 50 participants tested positive for Lactobacilli in the last trimester, and 14 of their newborns had Lactobacilli dominant mixed flora after birth. The researchers found that longer duration of ruptured membranes and IV antibiotics during labor were independent predictors of a decreased transmission of Lactobacillus to the baby. Of babies born to the 45 participants who tested positive for Lactobacilli- only 1 baby exposed to antibiotics during labor carried Lactobacillus at birth, while half of the newborns (16 in total) who were not exposed to IV antibiotics during labor did not have any Lactobacillus present at birth. Researchers are not sure why longer durations of ruptured membranes led to a decreased colonization with Lactobacillus. The researchers state that research is needed to determine if giving to probiotics to birthing people and newborns immediately after birth can help lessen the impact of IV antibiotics on the baby’s microbiome.

Aloisio et al. (2014)


Collected stool samples from 52 newborns in an Italian NICU at 6 to 7 days after birth. Half of these newborns were born to participants who had IV antibiotics for GBS, and the other half were born to participants who were GBS negative and did not receive antibiotics during labor. All the babies were born vaginally at term and were exclusively breastfed/chestfed. All types of bacteria that they tested for were present in both groups; however, the number (counts) of some types of bacteria differed between groups. In the group that was exposed to IV antibiotics during labor, there were lower counts of E. coli and Bifidobacterium, but there were no differences between groups in the average counts of Bacteroides fragilis group, lactobacilli, and C. difficile. In the newborns who were exposed to antibiotics during labor, there was less diversity of the Bifidobacterium that were present. The authors stated that the most important difference was a significant reduction in bifidobacteria counts (one type of beneficial bacteria) at 1 week of life in infants whose birthing parents received IV antibiotics.
Aloisio (2016) Collected stool samples at 7 days of age from 10 babies born to GBS-negative birthing people who did not receive antibiotics during labor and 10 babies born to GBS-positive birthing people who received antibiotics during labor. The 20 newborns in this study were in the NICU at the time their samples were collected and were all born vaginally at term and had been exclusively breastfed/chestfed. The authors used a special genetic (DNA) analysis to look at the genetic diversity of the bacteria, called “next generation sequencing.”

The infants whose parents had not received antibiotics had a more “rich” and complex microbial profile, while the babies whose birthing parent had received in-labor antibiotics had very poor diversity of their microbiome, higher levels of gram-negative bacteria, and a decrease in beneficial bacteria such as Bifidobacterium, Bacteroidetes, and Actinobacteria. There were higher levels of gram-negative bacteria, Enterobacteriaceae, and Proteobacteria in the IV antibiotics group.


The authors stated that there were clear differences in the gut microbiome between the two groups at 1 week of age. They also state that the changes in the microbiome are consistent with what one would expect after administering an IV antibiotic like ampicillin that mainly acts against gram-positive bacteria, leading to an over-abundance of gram-negative bacteria.

Azad (2016)


Researchers enrolled 198 parent-infant pairs from a large Canadian cohort study. The infants included in this sub-study had stool samples collected at 3 and 12 months, and complete data about antibiotic exposure during labor and after birth. Infants were separated into 4 groups: no antibiotic exposure during labor with vaginal delivery (57%), antibiotic exposure with vaginal birth (21%), antibiotic exposure with elective Cesarean (9%), and antibiotic exposure with unplanned Cesarean (13%). Cefazolin was used during Cesareans, and penicillin was used during vaginal births. Researchers also measured the presence and duration of exclusive breastfeeding/chestfeeding. They used high through-put genetic sequencing to determine the infant gut microbiome.

Among vaginal births, GBS was the most common reason for antibiotic use during labor (76%), followed by prolonged rupture of membranes (24%). Half (52%) of the infants were exclusively breastfed/chestfed at 3 months, and half (49%) were still receiving human milk at one year.

The results showed that the infant microbiome was influenced by antibiotic exposure, route of delivery, and breastfeeding /chestfeeding. At 3 months, infants exposed to antibiotics during labor had a decreased level of Bacteroidetes, as well as a decrease in microbiome richness, regardless of breastfeeding status. The most severe deficiencies happened among infants born by Cesarean. Infants born by Cesarean also had higher levels of Clostridium, Enterococcus, and Streptococcus. At one year of age, most of the differences were gone, although some differences remained in infants who were not breastfed/chestfed for at least 3 months.


This was the most thorough study so far in terms of measuring antibiotic exposure during labor and during the first year of life, as well as breastfeeding. Breastfeeding/chestfeeding had the beneficial effect of helping the microbiome recover after antibiotic exposure—the benefits increased with exclusive feeding with human milk and with an increased duration. Although by one year of age the gut seemed to have recovered in all infants who were breastfed/chestfed for at least 3 months, the researchers were not able to rule out long-term health effects from the initial microbiome changes before that. It is not surprising that Bacteroidetes (a type of beneficial bacteria) were suppressed at 3 months following antibiotic exposure during labor, given that this type of bacteria is sensitive to penicillin and cefazolin.

Corvaglia (2016)


Researchers enrolled 84 healthy term infants whose birthing parents had all been screened for GBS. They excluded infants born by Cesarean, who received antibiotics after birth, those admitted to the NICU, or those whose birthing parents had antibiotics before labor. Stool samples were collected at 7 and 30 days of life, and infants were separated into two groups: those who were exposed to antibiotics during labor for GBS, or those who were born to a parent who was GBS-negative and did not receive antibiotics during labor. Researchers also collected info on breastfeeding/chestfeeding. None of the parents gave their infants probiotics after birth. They researchers counted the number of these beneficial bacteria: Bifidobacterium, Lactobacillus, and B. fragilis (includes Bacteroides). The number of Bificobacterium was lower in the antibiotic group at 7 days of life, but there were no differences between groups at 30 days. There were no differences in Lactobacillus or B. fragilis group bacteria at either time point. There were higher counts of Bifidobacterium at 7 days among infants who did not receive antibiotics during labor and those who were exclusively breastfed/chestfed. By 30 days, there was no relationship between Bifidobacterium and feeding type or antibiotics. The authors concluded that the counts of these bacteria were only temporarily lowered by antibiotic use during labor. However, this study is limited by its use of simple bacterial counts of only 3 bacterial groups, and no examination of microbiome richness, although they did use molecular techniques that improves the accuracy of bacterial counts.

Mazzola (2016)


Researchers enrolled 26 infants from the NICU in a hospital in Italy. Infants could be in the study if they were born vaginally at term, had a normal weight at birth (2.5 to 4 kg) and their mothers were screened for GBS at 35-37 weeks. Stool samples were collected at 7 days and 30 days of life.  Infants were separated into 4 groups: breastfed/chestfed infants born to a GBS-negative parent who did not receive antibiotics during labor (“breast fed control”), breastfed/chestfed infants born to GBS positive birthing parent who received antibiotics during labor, mixed-fed infants born to GBS negative birthing people (“mixed fed control”), and mixed-fed infants born to GBS positive birthing people who received antibiotics during labor.

Among breastfed/chestfed infants, the researchers found a lower diversity of bacteria at 7 and 30 days in infants who received antibiotics during labor compared to the control group. The most prominent differences at day 7 were found with higher numbers of Enterobacteriacaea family (Escherichia) and lower numbers of Bifidobacterium. By day 30, the bifidobacteria had recovered, but there were still higher levels of Enterobacteriaceae in the infants exposed to antibiotics during labor.

Mixed-fed infants exposed to antibiotics had higher numbers of Proteobacteria, Firmicutes, Streptococcus, and bacteria from the Enterobacteriaceae family when compared to mixed fed control infants at 7 days. The control groups who were mixed-fed had higher levels of Actinobacteria and Bacteroidetes, and Bifidobacterium at 7 days. By day 30, the two mixed-fed groups looked more similar in terms of their microbiome.

There was less diversity and richness in the microbiome of breastfed/chestfed infants exposed to antibiotics during labor, compared to mixed-fed infants exposed to antibiotics during labor.


These researchers are not sure why the breastfed/chestfed infants exposed to antibiotics during labor fared worse in terms of richness and diversity of their microbiome profile, compared to mixed-fed infants exposed to antibiotics. They propose that perhaps there are effects of the antibiotics on human milk when the birthing person is exposed to antibiotics during labor, but this needs further study. They also state that we need research on probiotic supplements to help newborns balance the microbiome after antibiotic exposure during labor.
Nogacka (2017) Researchers collected samples from 40 infants in Spain at 2 days, 10 days, 1 month, and 3 months. All the 40 infants were born vaginally at term; 18 were exposed to IV penicillin during labor for GBS prevention, and 22 were not exposed. A total of 29/40 were breastfed/chestfed.

Levels of Proteobacteria decreased over time but were not statistically significant between groups. Infants exposed to penicillin during labor had higher levels of Firmicutes at 10 and 90 days. The non-exposed infants had higher levels of probiotics including Actinobacteria, Bacteroidetes, and others.

When they examined feeding patterns, they found that exclusively breastfed/chestfed infants who were not exposed to antibiotics had higher levels of Bacteroidetes at all time points and lower levels of Proteobacteria.

There was no statistical difference between groups in the presence of antibiotic resistant genes in the samples at 1 month.

This study provides additional evidence that breastfeeding/ chestfeeding can lessen the negative effects of antibiotics on the infant microbiome.

A larger sample size would be needed to accurately study the effects of antibiotics during labor on genes for antibiotic resistance (there was a trend but it did not reach statistical significance).


We would like to acknowledge Dr. Jessica Illuzzi, Associate Professor of Obstetrics, Gynecology, and Reproductive Sciences, and researcher who studies Group B Strep at Yale School of Medicine, for her expert review and assistance in writing the original article published in 2014. The 2022 revisions were reviewed by Ihotu Jennifer Ali, MPH and Shannon J. Voogt, MD.

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