What is a big baby?
The medical term for big baby is macrosomia, which literally means “big body.” Some experts consider a baby to be big when it weighs more than 4,000 grams (8 pounds 13 ounces) at birth, and others say a baby is big if it weighs more than 4,500 grams (9 pounds, 15 ounces). A baby is also called “large for gestational age” if its weight is greater than the 90th percentile at birth (Rouse et al. 1996).
How common are big babies?
Big babies are born to about 1 out of 10 women in the U.S. Overall, 8.7% of all babies born at 39 weeks or later weigh between 8 lbs 13 oz and 9 lbs 15 oz, and 1.7% are born weighing 9 lbs 15 oz or more (U.S. Vital Statistics). You can see the percentages listed separately below for women who are diabetic and not diabetic.
Among women with gestational diabetes, researchers have found a dose-response relationship between blood sugar at diagnosis (the glucola test) and the baby’s weight. This means that the higher your blood sugar, the more likely you are to have a baby who is large for gestational age (Metzger et al. 2008). However, women who receive treatment for gestational diabetes cut their chances of having a big baby in half (Landon, Spong et al. 2009).
What is routine care for suspected big babies in the U.S.?
Although big babies are only born to 1 out of 10 women, the 2013 Listening to Mothers Survey found that 2 out of 3 American women had an ultrasound at the end of pregnancy to determine the baby’s size, and 1 out of 3 were told that their babies were too big. In the end, the average birth weight of these suspected “big babies” was only 7 lbs 13 oz (Declercq, Sakala et al. 2013).
Of the women who were told that their baby was getting big, 2 out of 3 said their care provider discussed inducing labor because of the suspected big baby, and 1 out of 3 said their care provider talked about planning a C-section because of the big baby.
Most of the women whose care providers talked about induction for big baby ended up being induced (67%), and the rest tried to self-induce labor (37%). Nearly 1 in 5 women said they were not offered a choice when it came to induction—in other words, they were told that they must be induced for their suspected big baby.
When care providers brought up planning a C-section for a suspected big baby, 1 in 3 women ended up having a planned C-section. Two out of five women said that the discussion was framed as if there were no other options—that they must have a C-section for their suspected big baby.
In the end, care provider concerns about a suspected big baby were the 4th most common reason for an induction (16% of all inductions), and the 5th most common reason for a C-section (9% of all C-sections). More than half of all moms (57%) believed that an induction is medically necessary if a care provider suspects a big baby. So in the U.S., most women have an ultrasound at the end of pregnancy to estimate the baby’s size, and if the baby appears large, their care provider will usually recommend either an induction or an elective C-section. Is this approach evidence-based?
This approach is based on 4 major assumptions:
- Big babies are at higher risk for problems.
- We can accurately tell if a baby will be big.
- Induction keeps the baby from getting any bigger, which lowers the risk of C-section.
- Elective C-sections for big baby are beneficial and don’t have any major risks.
What is the evidence for these assumptions? Are they true? Let’s take a closer look at each one.
Assumption #1: Big babies are at higher risk for problems.
Reality #1: Big babies are at higher risk for temporarily getting their shoulders stuck, but difficulty giving birth to shoulders is unpredictable and permanent injuries are rare. A care provider’s suspicion of a big baby is more harmful than an actual big baby itself.
One of the main problems with big babies is shoulder dystocia (“dis toh shah”) which means difficulty birthing the baby’s shoulders. In articles that were written about shoulder dystocia in the 1960’s-1980’s, researchers frequently referred to shoulder dystocia as the “obstetrician’s greatest fear.”
Although big babies are at higher risk for shoulder dystocia, most cases of shoulder dystocia happen in smaller babies (Morrison et al. 1992). This is because there are many more small and normal size babies being born than big babies. Unfortunately, researchers have found that it is impossible to predict who will have shoulder dystocia and who will not (Foster et al. 2011).
In non-diabetic women, shoulder dystocia happens 0.65% of the time in babies that weigh less than 8 lbs 13 oz (6.5 cases out of 1,000 births), 6.7% of the time in babies that weigh between 8 lbs 13 oz and 9 lbs 15 oz (60 out of 1,000), and 14.5% of the time in babies that weigh 9 lbs 15 oz or greater (145 out of 1,000) (Rouse et al. 1996).
Rates of shoulder dystocia are much higher in Type I and Type II diabetes (2.2% of babies that weigh less than 8 lbs 15 oz, 13.9% of babies that weigh between 8 lb 15 oz and 9 lb 13 oz, and 52.5% of babies that weigh more than 9 lb 13 oz), and among women with gestational diabetes (Rouse et al. 1996).
I was not able to find exact numbers for the percentage of women with gestational diabetes who have shoulder dystocia, as the rates change depending on each woman’s blood sugars. However, we have strong evidence that treatment for gestational diabetes drastically cuts the chance of having a big baby and shoulder dystocia (To read more, click here).
Although most cases of shoulder dystocia can be managed by a care provider, it can sometimes result in a nerve injury in the baby called brachial plexus injury. A baby does not have to have shoulder dystocia in order to experience a brachial plexus injury. One-third of brachial plexus injuries happen when there wasn’t any shoulder dystocia. In fact, some cases of brachial plexus injury happen after elective Cesarean surgery (Rouse et al. 1996).
Some infants who have brachial plexus injury (7 out of 100) will end up with permanent nerve damage to the arm and shoulder. The remaining infants who have an injury will get better—sometimes on their own, and sometimes after therapy or surgery (Rouse et al. 1996).
Although shoulder dystocia can be scary when it happens, there are ways care providers can help prevent and manage a shoulder dystocia. For more information, read this article on shoulder dystocia by Midwife Thinking.
Could my baby die if it experiences shoulder dystocia? Deaths from shoulder dystocia are possible but extremely rare. In 1996, researchers looked at all the studies so far that had reported the rate of death due to shoulder dystocia. In 15 studies, there were 1,100 cases of shoulder dystocia and no deaths (a death rate of 0%). In 2 other studies, the rates of infant death were 1% (1 baby out of 101 “died at delivery,” possibly due to the dystocia) and 2.5% (1 infant died out of 40 cases of shoulder dystocia) (Rouse et al. 1996).
Can big babies cause any other problems?
It is possible that women who give birth to big babies are more likely to have severe perineal tears (3rd or 4th degree). However, research studies have found conflicting results. For example, one large study found no difference in 3rd and 4th degree perineal tears between women who had big babies and those who had normal size babies (Weissmann-Brenner et al. 2012). In contrast, another study of hospital births in California during 1995-1999 found a higher rate of 4th degree tears in big babies who were born vaginally (Stotland et al. 2004). However, 4th degree tear rates in this particular study were very high, even among normal weight babies (1.5%), and the authors did not describe how many women had episiotomies, which is a leading cause of 4th degree tears.
Overall, the risk of a severe tear (3rd or 4th degree) is low in most women (anywhere from 0.2% to 0.6%), whether or not you have a big baby (Weissmann-Brenner et al. 2012). Although having a big baby may be a risk factor for severe tears, severe tears are uncommon to begin with, and a big baby is nowhere near as big a risk factor as other things like vacuum and forceps delivery. To put it into perspective, having a big baby may increase the risk of a severe tear by 3 times (so if your baseline risk was 0.2%, it would increase to 0.6%), but a vacuum delivery increases the risk by 11 times (from 0.2% to 2.2%), and the use of forceps increases the risk by 39 times (from 0.2% to 7.8%) (Sheiner et al. 2005).
Women who give birth to big babies may be at higher risk for postpartum bleeding (hemorrhage). In one large study, researchers found that women who gave birth to babies who weigh more than 9 lbs 15 oz are more likely to have postpartum hemorrhage (1.7%) compared to women who had normal size babies (0.3%) (Weissmann-Brenner et al. 2012). However, it is not clear whether this higher rate of postpartum hemorrhage is due to the big baby itself or the inductions and C-sections that care providers often recommend for a big baby (Fuchs et al. 2013)—as both these procedures can increase the risk of postpartum hemorrhage (Magann et al. 2005).
Some women have said their doctors recommend C-sections for big babies because there is a higher risk of stillbirth. However, I was not able to find any research evidence to support this claim—no evidence suggests a higher risk of stillbirth in big babies of non-diabetic women. The risk of stillbirth has historically been higher in women with Type I or Type II diabetes. However, in recent years the stillbirth rate for women with Type I or Type II diabetes has drastically declined, due to improvements in how we manage diabetes during pregnancy (Gabbe et al. 2012). As far as gestational diabetes goes, the largest study ever done on gestational diabetes found no link between gestational diabetes and stillbirth (Metzger et al. 2008).
Perhaps most importantly, when a big baby is suspected, women are more likely to experience a harmful change in how their care providers see and manage labor and delivery. This leads to a higher C-section rate and a higher rate of women inaccurately being told that labor is taking too long or the baby does not “fit.”
In fact, research has consistently shown that the care provider’s perception that a baby is big is more harmful than an actual big baby by itself. In a very important study, researchers what happened to women who were suspected of having a big baby (>8 lbs 13 oz) to what happened to women who were not suspected of having a big baby—but who ended up having one (Sadeh-Mestechkin et al. 2008). The end results were astonishing. Women who were suspected of having a big baby (and actually ended up having one) had a triple in the induction rate; more than triple the C-section rate, and a quadrupling of the maternal complication rate, compared to women who were not suspected of having a big baby but who had one anyways.
Complications were most often due to C-sections and included bleeding (hemorrhage), wound infection, wound separation, fever, and need for antibiotics. There were no differences in shoulder dystocia between the 2 groups. In other words, when a care provider “suspected” a big baby (as compared to not knowing the baby was going to be big), this tripled the C-section rates and made mothers more likely to experience complications, without improving the health of babies (Sadeh-Mestechkin et al. 2008).
Other researchers have found that when a first-time mom is incorrectly suspected of having a big baby, that care providers have less patience with labor and are more likely to recommend a C-section for stalled labor. In this study, researchers followed 340 first-time moms who were all induced at term. They compared the ultrasound estimate of the baby’s weight with the actual birth weight. When the ultrasound falsely said the baby was going to weigh more than 15% higher than it ended up weighing, physicians were more than twice as likely to diagnose “stalled labor” and perform a C-section for that reason (35%) than if there was no overestimation of weight (13%) (Blackwell et al. 2009b).
Overall, a total of 8 different studies—spanning the years 1992 to 2012—have all shown that it is the suspicion of a big baby—not big babies themselves—that leads to higher induction rates, higher C-section rates, and higher rates of stalled labor (Levine et al. 1992;Weeks et al. 1995; Parry et al. 2000; Weiner et al. 2002;Sadeh-Mestechkin et al. 2008; Blackwell et al. 2009; AW et al. 2010; Little et al. 2012).
So although big babies are at higher risk for some problems, an even bigger problem for some women is the care provider’s perception that there is a big baby. This perception—whether it is true or false—changes the way the care provider behaves, which increases the chance of bad outcomes. For most women, an ultrasound at the end of pregnancy to estimate the baby’s size increases the chance of harm to her, and does not improve the health of the baby. As the title of one research study says, “Big baby?? Better not tell!”
Assumption #2: We can tell whether a baby will be big at birth.
Reality #2: Care providers and ultrasound are equally poor at predicting whether a baby will be big at birth.
Time and time again, researchers have found that it is very difficult to predict a baby’s size before it is born. Although 2 out of 3 U.S. women receive an ultrasound at the end of pregnancy (Declercq et al. 2013) to “estimate the baby’s size,” ultrasound results are very unreliable.
In 2005, researchers looked at all of the studies that had ever been done on ultrasound and estimating the baby’s weight at the end of pregnancy. They found 14 studies that looked at ultrasound and its ability to predict that a baby would weigh more than 8 lbs 13 oz. Ultrasound was only accurate 17% to 79% of the time, with most studies showing that the accuracy (“post-test probability”) was less than 50%. This means that for every 10 babies that ultrasound predicts will weigh more than 8 pounds, 13 ounces– 5 babies will weigh more than that and the other 5 will weigh less (Chauhan et al. 2005).
Ultrasound is even worse at trying to predict babies who will be born weighing 9 pounds 15 ounces or greater. In 5 studies that were done, the accuracy of ultrasounds to predict extra-large babies was only 20-30%. This means that for every 10 babies the ultrasound identified as weighing more than 9 pounds 15 ounces, only 2 to 3 babies actually weighed that much, while the other 7 to 8 babies weighed less (Chauhan et al. 2005).
The researchers found 4 studies that looked at the ability of ultrasound to predict big babies in women with diabetes. The accuracy of these ultrasounds was 61-63%, which means that for every 10 babies of diabetic women who are thought to weigh more than 8 lbs 13 oz, 6 will weigh that much and 4 will weigh less. The ultrasound test probably performs better in diabetic women simply because diabetic women are more likely to have big babies. In other words, it’s easier to predict a big baby in someone who is much more likely to have a big baby to begin with.
Care providers are equally inaccurate when it comes to estimating the size of the baby. When a care provider estimates that a baby is going to weigh more than 8 lb 13 oz, the overall accuracy is only 40-53%. This means that out of all the babies that are thought to weigh more than 8 lbs 13 oz, half will weigh more than 8 lbs 13 oz and half will weigh less. The care provider’s accuracy goes up if the woman has diabetes or is post-term, again, because the chance of having a big baby is higher among these women.
Assumption #3: Induction allows the baby to be born at a smaller weight, which helps avoid shoulder dystocia and reduces the risk of C-section.
Reality #3: Although suspected big babies who are induced are born about 3.5 ounces lighter, induction for suspected big baby does not lower the risk of shoulder dystocia, and it may increase the risk of C-section.
Researchers have consistently found that induction for suspected big babies does not improve the health of moms or babies. In a 2009 Cochrane review, researchers (Irion and Boulvain 2000) combined 3 studies in which 372 women with suspected big babies were randomly assigned to either 1) induction or 2) waiting for normal labor. When researchers compared the induction group to the normal labor group, they found no differences in C-section rates or shoulder dystocia rates. The researchers did not look at neonatal ICU admissions, Apgar scores, death rates, perineal tears, mothers’ satisfaction with care, or any long-term outcomes.
Because Gonen (1997) was the largest study included in the Cochrane review, let’s take a closer look at it. In this study, women were included if they were at least 38 weeks, had a suspected big baby (8 lbs 13 oz to 9 lbs 15 oz), did not have gestational diabetes, and had not had a previous C-section. Less than half the women were first-time moms. Women were randomly assigned (like flipping a coin) to either immediate induction with oxytocin (sometimes with cervical ripening) or waiting for normal labor.
The results? Women in the normal labor group went into labor about 5 days later than women who were immediately induced. Although women in the normal labor group tended to have slightly bigger babies (on average 3.5 ounces or 99 grams heavier), there was no difference in shoulder dystocia or C-section rates. All 11 cases of shoulder dystocia were easily managed without any nerve damage or trauma. Two infants in the normal labor group had mild brachial plexus injury—but neither of these 2 infants had shoulder dystocia, and both injuries were only temporary. Finally, researchers found that the ultrasound overestimated the baby’s weight 70% of the time and under-estimated the baby’s weight 28% of the time.
In summary, the researchers found that: 1) ultrasound estimation of weight was inaccurate, 2) shoulder dystocia and nerve injury were unpredictable, and 3) induction for big baby did not decrease the C-section rate or the risk of shoulder dystocia.
Although the randomized, controlled trials on induction for big baby found that induction did not hurt or help moms or babies, the overall number of women enrolled in those studies was small (less than 400 women). So it is helpful for us to look at observational studies, in which researchers followed large numbers of women with suspected big babies to see what happened.
In 2002, researchers combined the results of 9 observational studies that compared women who were induced for big baby and women who went into normal labor on their own (Sanchez-Ramos et al. 2002). Women who went into normal labor had a lower C-section rate (8% vs. 17%), a higher spontaneous vaginal birth rate (83% vs. 73%), and the same shoulder dystocia rate when they were compared to women who were induced. In other words, induction for suspected big baby increased the C-section rate and did not lower the shoulder dystocia rate.
There may be a couple of reasons why the observational studies found a higher C-section rate with induction, when the randomized, controlled trials did not. First of all, the number of women was much larger in the observational studies, which makes it easier to tell a difference in C-section rates. Second, women in the observational studies who were induced may have had other medical problems that made them more likely to end up with C-sections. Third, there were more first-time moms in the observational studies—and these moms would be more likely to end up a C-section if they are induced.
Assumption #4: Elective C-sections for big baby has benefits that outweigh the potential harms.
Reality #4: Among women who are not diabetic, it would take nearly 3,700 unnecessary C-sections to prevent one baby from having a permanent nerve injury due to shoulder dystocia. If care providers recommend an elective C-section for extra big babies (>9 lbs 15 oz), for every 3 cases of permanent nerve injury that they would prevent, there would be 1 extra maternal death.
Although some care providers will recommend an induction for a big baby, many skip this step and go straight to recommending an elective Cesarean. However, no studies have ever shown that a policy of elective Cesareans for big babies improves the health of moms and babies. On the contrary, researchers have estimated that this type of approach is extremely expensive and that it would take thousands of unnecessary C-sections to prevent one case of permanent nerve injury.
In 1996, a very important study published in the Journal of the American Medical Association found that a policy of elective C-sections for all big babies was not cost-effective and that there were more potential harms than potential benefits (Rouse et al. 1996).
In this study, the researchers calculated the potential effects of 3 different types of policies:
- No routine ultrasounds to estimate the baby’s size
- Routine ultrasounds and elective C-section for babies weighing 8 lbs 13 oz or more
- Routine ultrasounds and elective C-section for babies weighing 9 lbs 15 oz or more.
The researchers looked at the results separately for diabetic and non-diabetic women.
The results? Among non-diabetic women, a policy of elective C-sections for all suspected big babies over 8 lbs, 13 oz puts a large number of women and babies at risk of expensive and unnecessary surgeries. In order to prevent 1 permanent nerve injury, 2,345 women would have unnecessary C-sections at a cost of $4.9 million dollars per injury prevented (costs were estimated using year 1995 dollars).
With a policy of elective C-sections for all suspected big babies over 9 lbs 15 oz, even more women would have unnecessary surgery. In order to prevent 1 permanent nerve injury, 3,695 women would need to undergo unnecessary C-sections at a cost of $8.7 million per injury prevented.
The authors estimated that for every 3.2 permanent nerve injuries prevented, there would be 1 maternal death due to the increased risk of death associated with C-sections. Other possible harms associated with the elective C-section policy include higher rates of serious maternal infections, blood clot disorders, bleeding (hemorrhage) that requires blood transfusions, and newborn breathing problems.
Among diabetic women, the results were different—mostly because ultrasound is slightly more reliable at predicting big babies in women who are diabetic, and because shoulder dystocia is more common in these women as well. If diabetic women were offered an elective C-section for every baby that is suspected of weighing more than 8 pounds 13 ounces, it would take 489 unnecessary surgeries to prevent one case of permanent nerve damage. This would cost $930,000 per injury avoided. If diabetic women had an elective C-section when their babies were suspected of being 9 pounds 15 ounces or greater, it would take 443 unnecessary surgeries to prevent one case of permanent nerve injury, at a cost of $880,000 per injury avoided.
In summary, evidence does not support elective C-sections for all suspected big babies, especially among non-diabetic women. There have been no randomized, controlled trials testing this intervention. It is likely that for most non-diabetic women, the potential harms of an elective C-section for a big baby outweigh the potential benefits.
In fact, non-diabetic women may be given inaccurate information if their care providers present elective C-section as a completely “safe” or “safer” option than vaginal birth for a suspected big baby, when in fact Cesarean surgery carries much potential harm for the mom, infant, and children born in future pregnancies. To read more about the potential benefits and harms of C-section versus vaginal birth, you may want to read:
- “Why Cesareans are a BIG DEAL to you, your wife, and your daughter” from VBACFacts.com
- “C-sections: Best Evidence” from ChildbirthConnection.org
What is the bottom line? In summary, for non-diabetic moms:
- Ultrasounds and care providers are equally inaccurate at predicting whether or not a baby will be big. If an ultrasound or a care provider predicts a big baby, they will be wrong half the time.
- If a care provider thinks that you are going to have a big baby, this thought is more harmful than the actual big baby itself
- The suspicion of a big baby leads many care providers to manage a woman’s care in a way that triples her risk of C-section and quadruples the risk of complications.
- Because of this “suspicion problem,” ultrasounds to estimate a baby’s weight probably do more harm than good in most women.
- Induction for big baby does not lower the risk of shoulder dystocia and may increase the risk of C-section, especially in first-time moms
- A policy of elective C-sections for big babies likely does more harm than good for most women
- It would take nearly 3,700 elective C-sections to prevent one permanent case of nerve injury in babies who are suspected of weighing more than 9 pounds 15 ounces
- For every 3 permanent nerve injuries that are prevented, there will be 1 maternal death due to the elective C-sections
For diabetic moms and moms with gestational diabetes:
- Ultrasounds are slightly more accurate at predicting a big baby, but only because these moms are at higher risk of having a big baby to begin with
- Elective C-sections may be more cost-effective in women who have Type I or Type II diabetes
- Treatment for gestational diabetes drastically lowers the chance of having a big baby and shoulder dystocia
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Gestational diabetes and the glucola test (Click here)
Does gestational diabetes always mean a big baby and an induction? (Click here)
What is the evidence for erythromycin eye ointment in newborns? (Click here)
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Thank you to my 2 regular physician reviewers and to Amy Romano, MSN, CNM, Co-author of Optimal Care in Childbirth: The Case for a Physiologic Approach, for their feedback on this blog post. Thank you also to my readers for contributing photos of their big (and not so big), beautiful babies!
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