Rebecca Dekker

PhD, RN

 

The Evidence on: Due Dates

This article was originally published in 2015 and last updated on November 24, 2019 by Rebecca Dekker, PhD, RN and Anna Bertone, MPH. Please read our Disclaimer and Terms of UseFor a printer-friendly PDF, become a Professional Member to access our complete library.

What is an estimated due date, and how is it determined? What are the risks of going past your due date? Does induction increase the risk of Cesarean? At which point do the benefits of being electively induced outweigh the risks? Does a person’s goals and preferences for their births matter?

The purpose of this Evidence Based Birth® article is to look at the evidence on due dates.

How often are pregnant people induced for going past their estimated “due date?”

Inductions for non-medical reasons have been on the rise in the U.S. and around the world over the last 30 years (Little, 2017). Increasingly, more pregnant people are being induced because they have reached their estimated “due date” of 40 weeks.

According to the 2013 Listening to Mothers III survey, more than four out of ten mothers (41%) in the U.S. said that their care provider tried to induce labor (Declercq et al. 2013). The researchers asked mothers to select the reasons that they were induced.

Out of everyone who was induced, 44% said that they were induced because their baby was full term and it was close to the due date. Another 18% said that they were induced because the health care provider was concerned that the mother was overdue.

In the U.S., the Centers for Disease Control (CDC) reported that 27% of people were induced in 2018 (Martin et al. 2019). But that number is probably low. It’s likely that induction of labor is underreported in federal vital statistics (Declercq et al. 2013).

Some researchers define neonatal death as a death in the first week of life; others use the first month of life. Most of the research studies reviewed in this article used the definition of first week of life.

Why is there so much controversy about elective induction?

For many years, the common belief was that elective (not medically indicated) inductions doubled the Cesarean rate, especially in first-time mothers. Elective inductions might occur for social reasons, like the doctor wanting the mom to give birth before he or she goes out of town, or other non-medical reasons like the mother wanting to be done with an uncomfortable pregnancy. There is a gray zone around elective inductions. Many providers only consider the induction “elective” when it is with healthy women, single pregnancies, and at less than 41 weeks of pregnancy (Little, 2017). Otherwise, the induction may be considered a medically indicated induction. 

However, in the 2010s, some researchers began to dispute the claim that elective induction doubles the risk of cesarean. They argued that earlier studies—where elective induction showed a doubling in Cesarean rates—were flawed.

In the earlier studies, elective induction was compared only to spontaneous labor: people who were electively induced versus people who went into spontaneous labor. Excluded from these two groups were people who were not electively induced, but waited for labor and then ended up having medically indicated inductions later on (and, thus, a higher rate of Cesareans). For an example of this earlier flawed research, see this article by Yeast et al. 1999.

Previous studies compared cesarean rates of these two groups only:

New researchers pointed out that we need to compare people who have elective inductions with the whole group of those who wait for spontaneous labor—whether or not they actually do have spontaneous labor.

This is a subtle difference, but an important one, because not everyone who waits for labor will actually have a spontaneous labor; some of them will develop complications that lead to an induction and increase their risk for Cesarean. The researchers argued that the comparison group must include them as well.

This graphic shows how you would look at the two groups: the elective induction group versus the entire group of people who were not electively induced—some of whom would, in fact, end up being induced later for medical reasons.

Because of this flaw in the earlier studies, the researchers argued, we really can’t determine if elective induction between 39-41 weeks is better or worse than waiting for labor to start on its own, also called “expectant management.” 

What does it mean to be “full term?”

For many years, a baby was defined as being born at “term” if it was born between 37 weeks 0 days and 41 weeks 6 days. Anything before that 5-week period was considered “preterm,” and anything after those five weeks was “post-term.”

Over time, though, research began to show that health problems were more common at certain points during this 5-week “term” period. In particular, newborns are more likely to die (although the overall risk was still very low) if they are born before 39 weeks, or after 41 weeks.

The chance of a newborn having problems is lowest if he or she is born between 39 weeks and 0 days and 40 weeks and 6 days (Spong 2013).

In 2012, a group of experts came together to define “term” pregnancy. Based on their review of the research evidence, they broke the 5-week term period into separate groups (Spong 2013)

  • “Early term” babies are born between 37 weeks 0 days and 38 weeks 6 days
  • “Full term” babies are born between 39 weeks 0 days and 40 weeks 6 days.
  • “Late term” babies are born between 41 weeks 0 days and 41 weeks 6 days
  • “Post term” babies are born at 42 weeks and 0 days or later

How do you figure out your estimated due date?

Almost everyone—including doctors, midwives, and online due date calculators—uses Naegele’s rule (listen to the pronunciation here) to figure out an estimated due date (EDD).

Naegele’s rule assumes that you had a 28-day menstrual cycle, and that you ovulated exactly on the 14th day of your cycle (Note: some health care providers will adjust your due date for longer or shorter menstrual cycles).

To calculate your EDD according to Naegele’s rule, you add 7 days to the first day of your last period, and then count forward 9 months (or count backwards 3 months). This is equal to counting forward 280 days from the date of your last period.

For example, if your last menstrual period was on April 4 you would add seven days (April 11) and subtract 3 months = an estimated due date of January 11.

Another way to look at it is to say that your EDD is 40 weeks after the first day of your last period.

In cases where the date of conception is known precisely, such as with in vitro fetilization or fertility tracking where people know their ovulation day, the EDD is calculated by adding 266 days to the date of conception (or subtracting 7 days and adding 9 months). This increases the accuracy of the EDD because it no longer assumes a 28-day cycle with ovulation occurring on Day 14.

But where did Naegele’s rule come from?

In 1744, a professor from the Netherlands named Hermann Boerhaave explained how to calculate an estimated due date. Based on the records of 100 pregnant women, Boerhaave figured out the estimated due date by adding 7 days to the last period, and then adding nine months (Baskett & Nagele 2000).

However, Boerhaave never explained whether you should add 7 days to the beginning of the last period, or to the last day of the last period.

In 1812, a professor from Germany named Carl Naegele quoted Professor Boerhaave, and added some of his own thoughts. (This is how Naegele’s rule got its name!) However, Naegele, like Boerhaave, did not say when you should start counting—from the beginning of the last period, or the last day of the last period.

His text can be interpreted one of two ways: either you add 7 days to the first day of the last period, or you add 7 days to the last day of the last period.

As the 1800s went on, different doctors interpreted Naegele’s rule in different ways. Most added 7 days to the last day of the last period.

However, by the 1900s, for some unknown reason, American textbooks adopted a form of Naegele’s rule that added 7 days to the first day of the last period (Baskett & Nagele 2000).

And so this brings us to today, where almost all doctors use a form of Naegele’s rule that adds 7 days to the first day of your last period, and then counts forward 9 months—a rule that is not based on any current evidence, and may not have even been intended by Naegele.

What is the most accurate way to tell how far along you are?

Doctors started using ultrasound in the 1970s. Soon after, ultrasound measurement replaced last menstrual period (LMP) as the most reliable way to define gestational age (Morken et al. 2014).

A large body of evidence shows that ultrasounds done in early pregnancy are more accurate than using LMP to date a pregnancy. In a 2015 Cochrane review, researchers combined the results from 11 randomized clinical trials that compared routine early ultrasound to a policy of not routinely offering ultrasound (Whitworth et al. 2015). 

The researchers found that people who had an early ultrasound to date the pregnancy were less likely to be induced for a post-term pregnancy.

In other words, using the LMP to estimate your due date makes it more likely that you will be mislabeled as “post-term” and experience an unnecessary induction.

In a large observational study that enrolled more than 17,000 pregnant people in Finland, researchers found that ultrasound at any time point between 8 and 16 weeks was more accurate than the LMP. When ultrasound was used instead of a “certain” LMP (in other words, the mother is “certain” about the date she had her last period), the number of “post-term” pregnancies decreased from 10.3% to 2.7% (Taipale & Hiilesmaa 2001).

Why is LMP less accurate than using ultrasound?

There are several reasons why the LMP is usually less accurate than an ultrasound (Savitz et al. 2002; Jukic et al. 2013; ACOG 2017). LMP is less accurate because it can have these problems:

  • People can have irregular menstrual cycles, or cycles that are not 28 days
  • People may be uncertain about the date of their LMP
  • Many people do not ovulate on the 14th day of their cycle
  • The embryo may take longer to implant in the uterus for some people
  • Research indicates that some people are more likely to recall a date that includes the number 5, or even numbers, so they may inaccurately recall that the first day of their LMP has one of these numbers in it

What is the best time to have an ultrasound to determine gestational age?

In a 2013 study, researchers grouped ultrasound scans by <7 weeks, 7-10 weeks, 11-14 weeks, 14-19 weeks, and 20-27 weeks (Khambalia et al. 2013).

The authors found that the most accurate time to perform an ultrasound to determine the gestational age was 11-14 weeks. About 68% of people gave birth ±11 days of their estimated due date as calculated by ultrasound at 11-14 weeks. This was a more accurate result than any of the other ultrasound scans, and more accurate than the LMP.

The accuracy of the ultrasound saw a significant decline starting at about 20 weeks. Using an estimated due date from either the LMP or an ultrasound at 20-27 weeks led to a higher rate of pre- and post-term births.

Should a due date be changed based on a third trimester ultrasound?

In the Listening to Mothers III study, one in four mothers (26%) reported that their care provider changed their estimated due date based on a late pregnancy ultrasound. For 66% of the mothers, the estimated due date was moved up to an earlier date, while for 34% of the mothers, the date was moved back to a later date (Declercq et al. 2013).

Ultrasounds in the third trimester are less accurate than earlier ultrasounds or the LMP at predicting gestational age. Ultrasounds in the third trimester are not as accurate because they are measuring the size of the baby and comparing him or her to a “standard” sized baby. All babies are about the same size early in pregnancy. But if your baby will be larger than average, it will be perceived as “closer to done” when the ultrasound is done, and your due date will be moved up (incorrectly).

The reverse is also true for babies that will be smaller than average at term—their due date might be moved to a later date. This could be risky if the baby is experiencing growth restriction, as growth-restricted babies have a higher risk of stillbirth towards the end of pregnancy. Because of these problems with third trimester ultrasounds, the American College of Obstetricians and Gynecologists states that due dates should only be changed in the third trimester in very rare circumstances (2017).

They suggest that the due date should only be changed after a third trimester pregnancy ultrasound if 1) it is the pregnant person’s first ultrasound, and 2) it is more than 21 days different than the due date suggested by the LMP (ACOG 2017).

How long is a normal pregnancy? Is it really 40 weeks?

In the U.S. and other Western countries, induction is common at or even before 40 weeks, so it is impossible to know exactly what percentage of people today would naturally go into labor and give birth before, on, or after their estimated due date.

In the past, researchers figured out the average length of a normal pregnancy by looking at a large group of pregnant people, and measuring the time from ovulation (or the last menstrual period, or an ultrasound) until the date the person gave birth—and calculating the average. However, this method is wrong and does not give us accurate results.

Why is this method wrong?

This method does not work because many people are induced when they reach 39, 40, 41, or 42 weeks.

If you do include these induced people in your average, then you are including people who gave birth earlier than they would have otherwise, because they were not given time to go into labor on their own.

But this puts researchers in a bind, because if you exclude a person who was induced at 42 weeks from your study, then you are ignoring a pregnancy that was induced because it went longer—and by excluding that case, you artificially make the average length of pregnancy too short.

So how can we deal with this problem?

Researchers today use a method called “survival analysis” or “time to event analysis.” This is a special method that allows you to include all of these people in your study, and still get an accurate picture of how long it takes the average person to go into spontaneous labor.

There have been two studies that measured the average length of pregnancy using survival analysis:

Study finds that estimated due date is 3 to 5 days AFTER 40 weeks

In a very important study published in 2001, Smith looked at the length of pregnancy in 1,514 healthy women whose estimated due dates, as calculated by the last menstrual period, were perfect matches with estimated due dates from their first trimester ultrasound (Smith 2001).

The researchers found that 50% of all women giving birth for the first time gave birth by 40 weeks and 5 days, while 75% gave birth by 41 weeks and 2 days.

Meanwhile, 50% of all women who had given birth at least once before gave birth by 40 weeks and 3 days, while 75% gave birth by 41 weeks.

This means that for both first-time and experienced mothers in Smith’s study, the traditional “estimated due date” of 40 weeks was wrong!

The actual pregnancy was about 5 days longer than the traditional due date (using Naegele’s rule) in a first-time mother, and 3 days longer than the traditional due date in a mother who has given birth before.

Study finds that estimated due date should be closer to 40 weeks and 5 days

In 2013, Jukic et al. used survival analysis to look at the normal length of a pregnancy. This was a smaller study—there were only 125 healthy women, and they all gave birth between the years 1982 and 1985. However, this was also an important study, because researchers followed the participants even before conception and measured their hormones daily for six months (Jukic et al. 2013).

This means that the researchers knew the exact days that the participants ovulated, conceived, and even when their pregnancies implanted!

So what was the average length of a pregnancy in this study?

After excluding women who had preterm births or pregnancy-related medical conditions, the final sample of 113 women had a median time from ovulation to birth of 268 days (38 weeks, 2 days after ovulation).

The median time from the first day of the last menstrual period to birth was 285 days (or 40 weeks, 5 days after the last menstrual period).

The length of pregnancy ranged from 36 weeks and 6 days to one person who gave birth 45 weeks and 6 days after the last menstrual period. The 45 weeks and 6 days sounds really long… but this particular person actually gave birth 40 weeks and 4 days after ovulation. Her ovulation did not fit the normal pattern, so we know her LMP due date was not accurate.

The researchers also found that:

  • 10% gave birth by 38 weeks and 5 days after the LMP
  • 25% gave birth by 39 weeks and 5 days after the LMP
  • 50% gave birth by 40 weeks and 5 days after the LMP
  • 75% gave birth by 41 weeks and 2 days after the LMP
  • 90% gave birth by 44 weeks and zero days after the LMP

Remember though, some of the participants did not ovulate on the 14th day of their period (that’s why you saw the statistic that 10% still haven’t given birth by 44 weeks after the LMP!) So if we look at when people give birth after ovulation, you’ll see this pattern:

  • 10% gave birth by 36 weeks and 4 days after ovulation
  • 25% gave birth by 37 weeks and 3 days after ovulation
  • 50% gave birth by 38 weeks and 2 days after ovulation
  • 75% gave birth by 39 weeks and 2 days after ovulation
  • 90% gave birth by 40 weeks and zero days after ovulation

Women who had embryos that took longer to implant were more likely to have longer pregnancies. Also, women who had a specific sort of hormonal reaction right after getting pregnant (a late rise in progesterone) had a pregnancy that was 12 days shorter, on average.

So is the traditional “due date” really your due date?

Based on best evidence, there is no such thing as an exact “due date,” and the estimated due date of 40 weeks is not accurate. Instead, it would be more appropriate to say that there is a normal range of time in which most people give birth. About half of all pregnant people will go into labor on their own by 40 weeks and 5 days (for first-time mothers) or 40 weeks and 3 days (for mothers who have given birth before). The other half will not.

If someone is worried about experiencing pressure from their friends to give birth by a certain time point, they may want to tell family and friends that they have a “guess date” or a “guess month,” and refrain from sharing any specific estimated due date.

Are there some things that can make your pregnancy longer?

By far, the most important predictor of a longer pregnancy is a family history of long pregnancies—including your own personal history, your mother and sisters’ history, and your male partner’s family history (Jukic et al. 2013; Oberg et al. 2013; Mogren et al. 1999; Olesen, et al. 1999; Olesen et al. 2003).

In 2013, Oberg et al. published a large study that looked at more than 475,000 Swedish births, most of which were dated with an ultrasound before 20 weeks. They found that genetics has an incredibly strong influence on your chance of having a birth after 42 weeks:

  • If you’ve had a post-term birth before, you have 4.4 times the chance of having another post-term birth with the same partner
  • If you’ve had a post-term birth before, and then you switch partners, you have 3.4 times the chance of having another post-term birth with your new partner
  • If your sister had a post-term birth, you have 1.8 times the chance of having a post-term birth

Overall, researchers found that half of your chance for having a post-term birth comes from genetics. This includes the baby’s genetic tendency to gestate longer (due to genes the baby inherited from the mother and the father), and the mother’s genetic tendency to carry a pregnancy longer.

The Swedish researchers even proposed that you could call some pregnancies “resistant,” because these mothers and/or fetuses have a genetically decreased tendency to start labor.

Other factors that may make your pregnancy more likely to go longer include:

What are the risks of going past your due date?

The risks of some complications go up as you go past your due date, and there are three studies that have shown us what the risks are.

  1. In 2003, Caughey et al. looked at 135,560 people who gave birth at term in California between the years 1995 and 1999 (Caughey et al. 2003). The participants in this sample all gave birth at Kaiser Permanente hospitals in northern California. The overall use of interventions (Cesareans and inductions) in this sample was not listed.
  2. In 2004, Caughey et al. looked at the records of 45,673 people who gave birth in a single hospital in California from 1992 to 2002 (Caughey & Musci 2004). The participants in this study were mostly well-educated. As far as intervention rates go, 18% gave birth by Cesarean and 16% with the help of vacuum or forceps. The rate of inductions was not listed.
  3. In 2007, Caughey et al. studied the medical records of 119,254 people who gave birth after 37 weeks at Kaiser Permanente between the years of 1995 and 1999. This was the same time period and same hospital as his 2003 study, but this time the researchers only looked at low-risk people who had health insurance. The overall Cesarean rate was 13.8%, and 9.3% gave birth with the help of vacuum or forceps. The authors also took whether or not people had inductions into account when they calculated the risks of going past your due date (Caughey 2007).

Risks for mothers:

  • The risk of chorioamnionitis (infection of the membranes) was lowest at 37 weeks (0.16%) and increased every week after that to a high of 6.15% at ≥ 42 weeks (Caughey et al. 2003)
  • The risk of endomyometritis (infection of the uterus) was lowest at 38 weeks (0.64%) and increased every week after that to a high of 2.2% at ≥ 42 weeks (Caughey & Musci 2004)
  • The risk of having a placenta abruption (placenta separates prematurely from the uterus) was lowest at 37 weeks (0.09%), and increased every week to a high of 0.44% at ≥ 42 weeks (Caughey et al. 2003)
  • The risk of preeclampsia was lowest at 37 weeks (0.4%) and highest at 40 weeks (1.5%), after which the risk did not change (Caughey et al. 2003)
  • The risk of postpartum hemorrhage was lowest at 37 weeks (1.1%) and increased almost every week to a high of 5% at 42 weeks (Caughey 2007)
  • The risk of a primary Cesarean (in people who have never had a Cesarean before) increased from 14.2% at 39 weeks to a high of 25% at ≥42 weeks (Caughey & Musci 2004)
  • The risk of having a primary Cesarean for a non-reassuring fetal heart rate was lowest at 37-39 weeks (13.3-14.5%) and reached a high of 27.5% at 42 weeks (Caughey 2007)
  • The risk of receiving forceps or vacuum assistance increased from 14.1% at 38 weeks to a high of 18.5% at 41 weeks (Caughey & Musci 2004)
  • The risk of having a 3rd or 4th degree tear was lowest at 37 weeks (3.4%) and increased every week to a high of 9.1% at 42 weeks. However, these numbers are much higher than are typically seen, and are partially related to the high use of vacuum and forceps in this study.

In their 2007 study, Caughey et al. reported that high use of induction, Cesareans, and vacuum/forceps for people with increasing gestational age may contribute to an increase in maternal risks. However, when the researchers used a statistical method to control for the use of interventions, the risks still increased with gestational age.

Risks for babies:

  • The risk of moderate or thick meconium increased every week starting at 38 weeks, and peaked at ≥42 weeks (3% at 37 weeks, 5% at 38 weeks, 8% at 39 weeks, 13% at 40 weeks, 17% at 41 weeks, and 18% at >42 weeks) (Caughey & Musci 2004)
  • Neonatal intensive care unit (NICU) admission rates were lowest at 39 weeks (3.9%) and rose to 5% at 40 weeks and 7.2% at ≥42 weeks (Caughey & Musci 2004)
  • The risk of the baby being large at birth (>9 lbs 15 oz or >4500 grams) rose starting at 38 weeks (0.5%), and doubled every week after that up until 42 weeks (6%) (Caughey & Musci 2004)
  • The odds of having a low 5-minute Apgar score went up starting at 40 weeks and increased each week until ≥42 weeks (exact numbers not reported; Caughey & Musci 2004)

Other risks for post-term pregnancy include having low fluid, and something called dysmaturity syndrome (growth restriction plus muscle wasting), which happens in about 10% of babies who go past 42 weeks.

For more information about meconium, see this article by Midwife Thinking about meconium stained waters.

What about the risk of stillbirth?

In this section, we will talk about how the risk of stillbirth increases towards the end of pregnancy.

There are two very important things for you to understand when learning about stillbirth rates in post-term pregnancies.

First, there is a difference between absolute risk and relative risk.

Absolute risk is the actual risk of something happening to you. For example, if the absolute risk of having a stillbirth at 41 weeks was 1.7 out of 1,000, then that means that 1.7 mothers out of 1,000 (or 17 out of 10,000) will experience a stillbirth.

Relative risk is the risk of something happening to you in comparison to somebody else. If someone said that the risk of having a stillbirth at 42 weeks compared to 41 weeks is 94% higher, then that sounds like a lot. But some people may consider the actual (or absolute) risk to still be low—1.7 per 1,000 versus 3.2 per 1,000.

Yes—3.2 is about 94% higher than 1.7, if you do the math! So, while it is a true statement to say “the risk of stillbirth increases by 94%,” it can be a little misleading if you are not looking at the actual numbers behind it.

The second important thing that you need to understand is that there are different ways of measuring stillbirth rates. Depending on how the rate is calculated, you can end up with different rates.

How do you measure stillbirth rates?

Up until the 1980s, some researchers thought that the risk of stillbirth past 41-42 weeks was similar to the risk of stillbirth earlier in pregnancy. So, they did not think there was any increase in risk with going past your due date.

However, in 1987, a researcher named Dr. Yudkin published a paper introducing a new way to measure stillbirth rates. Dr. Yudkin said that earlier researchers used the wrong math when they calculated stillbirth rates—they used the wrong denominator! (Yudkin, Wood et al. 1987).

Screen Shot 2017-09-07 at 5.55.18 PM

Here’s why this formula is wrong: We don’t need to know how many stillbirths happen out of every 1,000 births at 41 weeks. Instead, we need to know how many stillbirths happen at 41 weeks compared to all pregnancies and births at 41 weeks. In other words, you have to include the healthy, living babies that have not been born yet in your denominator.

When researchers began using this new formula to figure out stillbirth rates, they found something very surprising—the risk of stillbirth decreased throughout pregnancy, until it reached a low point at 37-38 weeks, after which the risk started to rise again.

Screen Shot 2017-09-07 at 5.55.24 PM

This finding—that the risk of stillbirth decreases throughout pregnancy, and then increases sometime after 37-38 weeks—has been found many times by different researchers in different countries. This phenomenon is called the “U-shaped curve” of stillbirth. In other words, there are higher rates of stillbirth earlier in pregnancy, then they go down until around 37-38 weeks, after which they rise again.

Because the risk of stillbirth starts to go up even more at 40, 41, and 42 weeks, some researchers argue that although 40 weeks and 3-5 days may be the physiological length of pregnancy, 40 weeks may be the functional length of a pregnancy.

In other words, the average pregnancy normally lasts about 40 weeks and 5 days, but in some researchers’ opinion, because of the increased risk of stillbirth and newborn death; 40 weeks may be as long as a pregnancy should go.

And although the stillbirth rates may seem low overall, if you happen to be a parent who experiences the 3 in 1,000 event at 42 weeks, then the risk doesn’t seem so low anymore.

Actual stillbirth rates vs. open-ended stillbirth rates

Even after researchers began using the new way of calculating stillbirth rates, there was still controversy about the best way to calculate this new formula for measuring stillbirth rates.

Different than what Yudkin proposed in 1987, some researchers preferred an “open-ended” stillbirth rate (also known as the “prospective risk of stillbirth”). An open-ended stillbirth rate at 40 weeks would tell us what a pregnant person’s risk of stillbirth was for any time after 40 weeks, if she let the pregnancy continue indefinitely.

Other researchers argued that most people (and doctors!) don’t want to know what the risk of stillbirth would be if a pregnant person chose to let the pregnancy continue on and on! (Hilder et al. 2000). They just want to know what the risk would be if they waited one more week until the next appointment, or even a few days.

But the “open-ended” stillbirth rate tells you what your risk of stillbirth at 40 weeks would be if you include babies born not just at 40 weeks, but 41 weeks, 42 weeks, 43 weeks, and on! (Boulvain et al. 2000).

In the end, you will find that stillbirth rates vary from study to study, depending on whether the researchers report the actual stillbirth rate, or the open-ended stillbirth rate.

So what is the risk of stillbirth as you go past your due date?

Since the late 1980’s, there have been at least 12 large studies that looked at the risk of stillbirth during each week of pregnancy. Some of the researchers used open-ended stillbirth rates, and some of them used actual stillbirth rates.

All of the researchers found a relative increase in the risk of stillbirth as pregnancy advanced.

To get an accurate picture of stillbirth in people who go past their due date, it would be best to look at studies that took place in more recent times. I’ve chosen 3 of the most recent studies to show you from Norway, Germany, and the U.S. To see all of the other studies, click to view the entire table here.

All 3 of these studies used the actual stillbirth rate—not the open-ended stillbirth rate. Two studies used ultrasound to calculate gestational age, and one study used the LMP.

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The largest meta-analysis to date on risks of stillbirth and newborn death at each week of term pregnancies was published in 2019 (Muglu et al. 2019). A meta-analysis is when researchers take multiple studies and combine all the data together into one big “meta” study. The researchers included 13 studies (15 million pregnancies, nearly 18,000 stillbirths).

The risk of stillbirth per 1,000 was 0.11, 0.16, 0.42, 0.69, 1.66, and 3.18 at 37, 38, 39, 40, 41, and 42 weeks of pregnancy, respectively. Based on their data, Muglu et al. (2019) calculated the “number needed to harm” by waiting for labor for one more week in order to experience one additional stillbirth. To experience one additional stillbirth, there would need to be at least 2,367 people waiting for labor for one more week starting at 39 weeks. At 40 weeks, 1,449 people would have to wait for labor for one more week to experience one additional stillbirth. At 41 and 42 weeks, only 604 and 315 people, respectively, would have to wait for labor for one more week to experience one additional stillbirth.

The researchers found more evidence that health care systems are failing Black mothers and babies—an alarming but common theme in health care research. Black mothers were 1.5 to 2 times more likely than White mothers to have a stillbirth at every week of pregnancy.

When they looked only at low-risk pregnancies, the risk of stillbirth was 0.12, 0.14, 0.33, 0.80, and 0.88 at 38, 39, 40, 41, and 42 weeks of pregnancy. Low-risk pregnancy was defined as pregnancies with a single baby, no congenital abnormalities, and no medical conditions in the mother.

There was no additional risk of newborn death when giving birth between 38 and 41 weeks, but newborn death did increase when pregnancies went beyond 41 weeks.

So, although most researchers have found an increase in stillbirth rates in the late term and post term period, some might consider the “absolute” increase in risk to be small until about 41 weeks, after which it reaches about 0.80-1.66 out of 1,000, depending on the mother’s risk factors for stillbirth.

Does anything increase the risk of stillbirth in an individual who is past 41 or 42 weeks?

Researchers have found several factors are related to a higher risk of stillbirth:

Post-term babies who are small for gestational age (body weight <10th percentile) have a 6-7 times higher chance of stillbirth and newborn death than post-term babies who are not small for gestational age.

Also, small for gestational age babies are often growth restricted at the 18-week ultrasound. So, the gestational age for these babies is often under-estimated.

This means that babies who are small for gestational age may be more post-term than we realize they are—increasing their risk while also leaving us unaware of their true gestational age (Morken et al. 2014).

Other factors that do not necessarily cause stillbirth but may increase the risk of stillbirth, in general, include:

*Racism, including the effects of prejudice and institutional racism, can increase the risk of poor outcomes, including stillbirth, in certain populations (Giscombe and Lobel, 2005)

Of course, parents can still experience the stillbirth of a child even when none of these risk factors are present. As many as a third of all stillbirths that take place before labor have no known cause (Warland & Mitchell 2014). To read more about theories of unexplained stillbirth, read this article here.

In the first study of its kind, researchers in Australia collected placentas with consent from 34 people between 37-39 weeks of pregnancy, 28 people between 41-42 weeks, and 4 people who experienced stillbirths between 32 and 41 weeks (Maiti et al. 2017). The placentas from 41-42 week pregnancies and from the stillbirths showed increased signs of aging, with decreased ability to transport nutrients to the baby and waste products away from the baby, compared to the placentas from the earlier term births. The rate of aging of the placenta varied in different pregnancies, so not all of the 41-42 week placentas showed signs of aging. Interestingly, the authors say that in the future it may be possible to predict which babies are at increased risk of stillbirth by measuring markers of placental aging in the mother’s blood. You can watch a 10-minute video describing their findings here.

Induction at 39 weeks versus waiting for labor

When someone gets closer or past their due date, they will often face the question about whether to induce labor or wait for labor to start on its own.

Inducing labor is also known as “active management.”

Waiting for labor to start on its own, usually with fetal testing to monitor the baby’s status, is called “expectant management.”

Many researchers have tried to compare the risks and benefits of induction versus expectant management for pregnant people who get close to or go past their due dates.

Cautions about the evidence

Before we begin discussing the evidence, it is important to note that there are some major drawbacks to the evidence that we have so far on induction versus waiting for labor to start:

  1. Many of the clinical trials were carried out in countries or time periods with low Cesarean rates. Some research results may not apply to hospitals with high Cesarean rates. Does your hospital have high rates of “failed inductions,” and strict time limits on the length of labor? If so, then this evidence may not apply to you, because induction may be more risky (more likely to lead to a Cesarean) in your specific hospital!
  2. In these studies, participants were randomly assigned to induction or to waiting for labor to start on its own. However, participants assigned to wait for labor do not always go into labor spontaneously. Sometimes they get induced for medical complications that develop while waiting for labor. Likewise, participants assigned to elective induction do not always get induced. For example, in the Hannah PostTerm trial (the biggest study about induction for post-dates), about a third of mothers who were assigned to the induction group actually went into labor spontaneously before the induction. Most researchers only report the Cesarean rates of the two study groups (those who were assigned to expectant management and active management), but it’s also important for us to look at the Cesarean rates in people who were actually induced or who actually went into spontaneous labor.
  3. In most studies, people in the expectant management group had many fetal tests, some of which may have showed possible signs of distress, and some of which were false positives (Minticoglou & Hall, 2002). This could have led to higher rates of Cesarean section for fetal distress during labor in the expectant management group, especially among those people in the expectant management group who were induced for fetal indications (Woods et al. 2013). Another researcher said, “It may be that the results of our review reflect doctors’ discomfort with delayed delivery in high-risk people that, once they are in labor, manifests as more frequent Cesarean sections: an example of research confirming the biases of the health care community” (Woods et al., 2013, pg. 682).
  4. The induction protocols varied from study to study, and even within studies themselves. For example, in the most influential study of all—the Hannah PostTerm study—people in the induction group first received drugs to ripen the cervix, and then drugs to induce labor. Meanwhile, people in the expectant management group who ended up being induced did NOT have cervical ripening. This put the expectant management group at a disadvantage because many of them had inductions without cervical ripening treatment—increasing their risk for a Cesarean.

The ARRIVE study of 39-week inductions

Researchers carried out the ARRIVE study (A Randomized Trial of Induction Versus Expectant Management) to find out if elective induction of labor during the 39th week of pregnancy would result in a lower rate of death and serious complications for babies, compared to waiting until at least 40 weeks and 5 days for elective induction (Grobman et al., 2018). They also wanted to see if inductions had an effect on the risk of Cesareans.

This was a large study that took place at 41 hospitals in the United States. Researchers screened more than 50,000 people to see if they could take part in the study. People had to be giving birth for the first time with a single, head-down baby, be certain of the date of their last menstrual period, and have no major medical conditions.

They found 22,533 people who were eligible to be in the study, but only 6,106 of them (27%) agreed to participate. Thus, there may have been selection bias, where the study’s findings among the trial participants may not reflect the overall eligible population.

The researchers randomly assigned (like flipping a coin) 3,062 people to be induced at 39 weeks, and 3,044 people to expectant management. Expectant management meant you could wait for labor to begin on its own as long as birth occurred by 42 weeks and 2 days, or be induced for medical reasons at any time, or be induced electively after 40 weeks and 5 days. In other words, people in the expectant management group experienced a mix of spontaneous labor, induced labor for medical reasons, and electively induced labor. Some people may wonder why the researchers did not simply compare elective induction with spontaneous labor. They could not compare those two groups, because spontaneous labor is not a certainty–it is possible someone may change their mind and wish to be induced electively, or require an induction for medical reasons.

What did the ARRIVE trial find?

They found that inducing labor at 39 weeks did not improve the primary outcome of death or serious complications for babies. Unfortunately, since stillbirths and newborn deaths are very rare at 39 and 40 weeks, the ARRIVE study (with 6,000 participants) was too small to tell if elective induction has an effect on this outcome. More babies received breathing support after expectant management (4.2% versus 3%) and had longer hospital stays, both of which could have been due to the higher rate of Cesareans with expectant management. 

For mothers, induction at 39 weeks was linked to a lower rate of Cesarean compared to those assigned to expectant management (19% Cesarean rate versus 22%) and a lower chance of developing pregnancy-induced high blood pressure (9% versus 14%). It’s worth noting that the participants in this study developed high blood pressure after 38 weeks of pregnancy at unusually high rates, and researchers have questioned if the decrease in Cesareans with elective induction was mostly because of the mothers who got high blood pressure at the end of pregnancy during expectant management (Carmichael and Snowden, 2019). The mothers in the early induction group spent more time in the hospital in labor, but less time in the hospital postpartum. There was no difference in the rate of mothers with ‘any breastfeeding’ (31% both groups) or ‘exclusive breastfeeding’ (33% both groups) 4 to 8 weeks after the birth. 

Although this study may be helpful with making informed decisions, it does not mean “everyone” should be induced. The ARRIVE study did find that inducing low-risk, first-time mothers with accurately estimated due dates at 39 weeks may help to lower the Cesarean rate from 22% to 19% if care providers follow the same induction practices as they did in this study. The researchers think this is because the risk of Cesarean goes up the longer a pregnancy continues. Longer pregnancies mean more opportunities for potential complications to show up and an increasing willingness by providers to perform a Cesarean.

The ARRIVE study does not mean that elective induction at 39 weeks lowers the risk of Cesarean for every individual. Some mothers may not benefit from early elective induction, including:

  • Those who prefer to avoid medical interventions. Many mothers would prefer to wait for labor to start on its own, if possible. This could be why so many people (73%) refused to participate in the study (although some may have refused because they knew they wanted early induction and didn’t want to wait). Some mothers want to avoid cervical ripening drugs, synthetic oxytocin, or mechanical induction with a Foley catheter, where an object presses against the cervix to help start labor. They may also want to avoid other medical interventions that go along with induction, such as intravenous fluids, continuous fetal monitoring, and restrictions on freedom of movement.
  • Those whose care providers have high Cesarean rates with inductions. In the ARRIVE study, providers knew they were participating in a research study looking at Cesarean rates, which can lower their Cesarean rate because they know they’re being “watched.” In this study, it was recommended that providers follow best practices for induction, such as using cervical ripening for anyone who had an unfavorable cervix. The researchers also recommended that mothers be given at least 12 hours in early labor before diagnosing a “failed” induction and ordering a Cesarean. Most providers in this study probably did follow these strict labor guidelines, because they were able to get a Cesarean rate of 19% with early induction in first-time mothers—this rate is unusually low, and not typical in many hospitals. The average Cesarean rate after induction among low-risk, first-time mothers giving birth in 240 California hospitals was 32%, with some rates as high as 60% (Main and CMQCC, 2018).
  • Those choosing midwifery care. Most of the people in this study were cared for by physicians (94%). Studies show that midwives achieve low rates of Cesarean without the regular use of elective induction. In the U.S., the Cesarean rate is about 5% at planned home births and 6% at midwifery-led birth centers (Cheyney et al., 2014; Stapleton et al., 2013). Hospitals with a higher percentage of midwife-attended births also tend to have lower rates of Cesarean; a recent study found a 15% Cesarean rate for hospitals that had more than 40% of their births attended by midwives (Attanasio and Kozhimannil, 2018).

An important limitation to the ARRIVE trial is that it was not designed to look at the practical implications of inducing everyone at 39 weeks. For example, what might happen to someone who has a serious medical need for an induction, but can’t get on the schedule because all of the hospital beds are full of people being electively induced at 39 weeks? 

Other ways to lower your risk of Cesarean besides elective induction

The ARRIVE trial reported that people assigned to elective induction at 39 weeks had a Cesarean rate of 19% compared to a rate of 22% among those assigned to expectant management. That was the absolute risk of having a Cesarean, or how often Cesareans actually happened in each group. Absolute risk is the actual, or true risk of something happening to you. Relative risk is the risk of something happening to you in comparison to someone else, and you have to carry out a math formula to understand the reduction in relative risk. The relative risk of having a Cesarean was 16% less in the early induction group compared to the expectant management group.

Although the relative risk reduction was 16% with elective induction, studies have found larger reductions in the relative risk of Cesarean using other approaches. People randomly assigned to continuous support during labor (such as with a doula) were 25% less likely to have a Cesarean (Bohren et al., 2017). Also, when people are assigned to a less-invasive type of fetal monitoring called hands-on listening (also known as intermittent auscultation), they are 39% less likely to have a Cesarean compared to people assigned to continuous electronic fetal monitoring (Alfirevic et al., 2017). Other comfort measures, such as walking around during labor, or planning a waterbirth, have also been shown in randomized trials to lower your risk of Cesarean by more than 16%. So, there are plenty of alternatives for people who want to lower their risk of Cesarean, but don’t want an elective induction.

Other randomized, controlled trials in recent years (much smaller than the ARRIVE trial)

Miller et al. (2015) conducted a trial at a U.S. military medical center. They randomly assigned 162 first-time mothers with an ‘unfavorable cervix’ to induction at 39 weeks (with cervical ripening and Pitocin®) or waiting for labor until no later than 42 weeks.

Of the people assigned to induction at 39 weeks, 79/82 people followed their assignment and were induced at 39 weeks (96%). Of the people assigned to expectant management, 79/80 followed their assignment (99%), meaning that they weren’t electively induced at 39 weeks; however, 44% gave birth after spontaneous labor and 56% gave birth after induction for medical reasons.

They found no difference in the rate of Cesareans between groups. To put it another way, elective induction at 39 weeks was not found to significantly increase or decrease the Cesarean rate. There was a high rate of Cesarean for labor arrest in the induction group (72% of Cesareans vs. 36% in EM group), which suggests that it is important to have a protocol for “failed” induction that aims to prevent unnecessary Cesareans. In the expectant management group, 13% of mothers were induced for high blood pressure disorders versus 0% of mothers in the 39-week induction group. As the pregnancy progresses, there are more opportunities for complications to develop.

Another randomized trial by Walker et al. (2016) assigned about 600 mothers from 42 hospitals in the United Kingdom to either inducing labor between 39 weeks 0 days and 39 weeks 6 days, or not inducing at 39 weeks and instead waiting up until 41-42 weeks before being induced. All of the participants in this study were over 35 years of age, so they called it the 35/39 trial. You can read more about this trial in our EBB® Signature Article on Advanced Maternal Age here. In brief, there was no difference in Cesarean rates between the induction at 39 weeks group and the not-induced-at-39-weeks group. There was no difference in any of the other birth complications for mothers or babies.

Retrospective studies in recent years

We found five retrospective studies conducted in the last five years that compared 39-week elective induction with expectant management. A retrospective study is one that looks back at events that took place in the past. Here, we’re focusing on studies that compared 39-week elective induction with expectant management, not studies with inductions later in pregnancy, or those that grouped 39-41 week inductions.

Four of the studies found a lower Cesarean rate with elective induction at 39 weeks compared to expectant management and one study found no difference in the Cesarean rate between groups. All five of the studies found newborn benefits with elective induction at 39 weeks. The largest retrospective study (California data from over 360,000 births, Darney et al. 2013) found lower perinatal death with elective induction at 39 weeks (0% versus 0.2%). However, these studies are not randomized, so they have inherent flaws. For example, although the researchers tried to take all of the important factors into account, it may be that the people considered the most likely to have a successful induction (the most healthy) were the ones electively induced.

Induction at 39 weeks

Pros

  • Avoid potential complications of continuing the pregnancy (e.g., developing a high blood pressure disorder, having a large baby)
  • Lower Cesarean rate with first-time mothers under best practice “failed” induction protocol to prevent unnecessary Cesareans
  • May prevent potential future stillbirth (although some would consider the absolute risk to be low until 41 weeks)
  • Convenience, end an uncomfortable pregnancy

Cons

  • Potential for failed induction leading to a Cesarean
  • Longer time spent in labor
  • Medicalization of birth (cascade of interventions)
  • Medically induced contractions (↑ pain)
  • Potential uterine tachysystole (more than 5 contractions in 10 minutes, averaged over a 30-minute window; it can lead to a possible decrease in oxygen to the baby and fetal heart rate changes)
  • Increased risk of infection (with some methods)
  • Unknown impact on L&D costs/resources

Late and post-term induction versus waiting for labor

In a recent Cochrane meta-analysis, researchers compared people who were electively induced to those who waited for labor to start on its own (Middleton et al. 2018). They included 30 randomized, controlled trials (over 12,000 mothers) comparing a policy of induction at or beyond term versus expectant management. The trials took place in Norway, China, Thailand, the U.S., Austria, Turkey, Canada, the U. K., India, Tunisia, Finland, Spain, Sweden and the Netherlands.

Most of the data (about 75%) came from trials of induction ≥41 weeks. This meta-analysis came out too early to include the large ARRIVE trial of 39-week induction versus expectant management or the other two large trials (INDEX and SWEPIS) that we will discuss below. The Hannah Post-Term trial, which we will describe in detail, was the largest trial included. They considered the overall evidence to be moderate quality.

What did they find? A policy of induction was linked to 67% fewer perinatal deaths compared to expectant management (2 deaths versus 16). The number needed to treat was 426 people with induction to prevent 1 perinatal death. Specifically, there were fewer stillbirths with a policy of induction (1 stillbirth versus 10). The absolute risk of perinatal death was 3 deaths per 1,000 births with a policy of expectant management versus 1 death per 1,000 births with a policy of induction.

A policy of induction was also linked to fewer Cesareans compared to expectant management (16.3% versus 18.4%).

Fewer babies assigned to induction had Apgar scores less than 7 at 5 minutes compared to those assigned to expectant management. There were no differences between groups in the rate of forceps/vacuum birth, perineal trauma, excessive bleeding after birth, total length of maternal hospital stay, newborn intensive care admissions, or newborn trauma.

They were not able to find differences between timing of induction (< 41 weeks versus ≥ 41 weeks) or by the state of the cervix for perinatal death, stillbirth, or Cesarean. The authors concluded that individualized counseling might help pregnant people choose between elective induction at or beyond term or continuing to wait for labor, and that providers must honor their values and preferences. We need more research to know who would or would not benefit from elective induction and the optimal time for induction is still not clear from the research.

The famous Hannah “Post-Term” study

We’ve mentioned it before, but the most important study that has ever been done on inducing for post-dates is the Hannah et al. 1992 Post-Term study. This study was published in the New England Journal of Medicine.

Because it was such a large study, the Hannah Post-Term study controls most of the findings in the Middleton et al. (2018) meta-analysis described above.

So, let’s look at what happened in this study

Between the years of 1985 to 1990, a group of researchers enrolled 3,407 low-risk pregnant people from six different hospitals in Canada into the Hannah Post-Term study.

People could be included if they had a live, single fetus, and were excluded if they were ≥ 44 weeks, were already 3 or more centimeters dilated, had a previous Cesarean, had pre-labor rupture of membranes, or had a medical reason for induction. The study took place in the six Canadian hospitals between the years 1985 and 1990.

At around 41 weeks, participants were randomly assigned to either induction of labor or fetal monitoring (expectant management).

In the induction group:

  • Labor was induced within four days of entering the study (usually about 4 days after 41 weeks)
  • If the cervix was not ripe (< 3 cm dilated and <50% effaced), and if the fetal heart rate was normal, participants were given prostaglandin E2 gel to ripen the cervix.
  • A maximum of 3 doses of gel were given every 6 hours. If this did not induce labor or if the gel was not used, participants were given IV oxytocin, had their waters broken, or both. They could not receive oxytocin until at least 12 hours after the last prostaglandin gel dose.

In the monitored (expectant management) group:

  • Participants were taught how to do kick counts every day and had nonstress tests 3 times per week.
  • The amniotic fluid level was checked by ultrasound 2-3 times per week.
  • Labor was induced if the nonstress test was nonreactive or showed decelerations, if there was low amniotic fluid (deepest pocket <3 cm), if complications developed, or if the mother did not go into labor on her own by 44 weeks.
  • If doctors decided that the baby needed to be born, mothers did not receive cervical ripening—instead, they either had their water broken and/or IV oxytocin, or had a Cesarean without labor.

What did researchers find in the Hannah Post-Term study?

In the induction group, 66% of people were induced, and 34% went into labor on their own before the induction. In the monitoring group, 33% were induced and 67% went into labor on their own.

The findings on Cesarean rates differ, depending on what numbers you look at.

You can look at the outcomes for the two original groups—the people randomly assigned to induction and those assigned to fetal monitoring—or you can look at the breakdown of what actually happened to the people in the two groups. In other words, what happened to the people who were actually induced or actually went into spontaneous labor?

What happened in the original, randomly assigned groups?

If you look at what happened in the two original groups (random assignment to induction and monitoring groups), the overall Cesarean rate was lower in the induction group (21.2% vs. 24.5%), even after taking into account whether this was the mother’s first baby, her age, cervical dilation at the time of study entry, and cervical dilation at the time of study entry

There was also a lower rate of Cesareans for fetal distress in the induction group vs. the monitoring group (5.7% vs. 8.3%).

But what happened to people who were actually induced or actually went into labor on their own?

So, if you break down those two groups—the induction and fetal monitoring groups—and look at what happened to the people who were actually induced, or who actually went into spontaneous labor, this is what you will see (Hannah et al. 1996):

So we see two very interesting things here: people who went into spontaneous labor, regardless of which group they were originally assigned, had a Cesarean rate of only 25.7%. But if people in the monitoring group had an induction, their Cesarean rate was much higher than all of the other groups—42%!

The same pattern holds true when you look at experienced mothers (people who had given birth before):

So what do these numbers mean?

Important details from the Hannah Post-Term study are hidden when you only look at the results according to random group assignment. The reported main findings were that a policy of fetal monitoring and expectant management increases the Cesarean rate.

But a closer look at the findings reveals that only the people who were expectantly managed but then had an induction had a really high Cesarean rate. People who were expectantly managed and went into labor spontaneously did NOT have higher Cesarean rates. One possible explanation for the high Cesarean rate seen in the people who were assigned to expectant management but then ended up getting an induction is that the people in this group may have been at higher risk for Cesarean to begin with, since a medical complication could have led to the induction. The people that were assigned to expectant management and never developed a complication requiring induction were the lower risk people, the ones less likely to give birth by Cesarean. Another factor that could have contributed to the high Cesarean rate in this group is the issue that we discussed previously—that doctors might have been quicker to call for a Cesarean when assisting the labors of people with medical inductions who had post-term pregnancies.

So if someone is considering expectant management after 41 weeks, one of the benefits is that if they go into labor on their own, they will have a relatively low risk of Cesarean. But one of the risks is that longer pregnancies mean more opportunities for potential complications to show up and if an induction becomes necessary, the risk of a Cesarean with that induction is nearly doubled, from 25.7% to 42%.

More evidence that 41-week induction might improve outcomes for babies

Two large trials on post-term induction have come out since the Middleton et al. (2018) Cochrane meta-analysis. 

The INDEX trial from the Netherlands

The trial from the Netherlands is called the INDEX trial, which stands for INDuction at 41 weeks, EXpectant management until 42 weeks (Keulen et al. 2019). It was a multicenter trial, conducted at 123 midwifery practices and 45 hospitals. They randomly assigned a total of 1,801 pregnant people to either induction at 41 weeks +0-1 days or to expectant management and induction at 42 weeks+0 days (if still no labor). In the Netherlands, it is not standard to induce labor before 42 weeks with an uncomplicated pregnancy, so they were able to get ethical approval to conduct this study.

Pregnant people were enrolled into the study between 2012 and 2016. Mothers had to be healthy and pregnant with single, head-down babies. Everyone had to have a gestational age that was estimated with ultrasound before 16 weeks of pregnancy. They excluded people with a prior Cesarean, high blood pressure disorders, expected problems with the baby’s growth, abnormal fetal heart rate, or known fetal malformations.

In both groups, cervical ripening and induction methods depended on local protocol. This is an important weakness of the study because, like the large Hannah Post-Term trial, the INDEX trial may have managed labor inductions differently based on group assignment.

In the elective induction group, 29% of the participants had spontaneous labor before their induction and 71% were induced. In the expectant management group, 74% of the participants went into labor spontaneously before their planned induction and 26% were induced. Interestingly, the median reduction in length of pregnancy between groups was only two days.

What did the INDEX trial find?

For mothers:

  • There was no difference in Cesarean rates (11% both groups).
  • There was no difference in a combined measure of bad outcomes called the composite adverse outcome rate (11%-14% both groups). This outcome included excessive bleeding after birth (≥1000 mL), and/or manual removal of placenta, and/or severe tears, and/or intensive care admission, and/or maternal death. No maternal deaths occurred in either group. 

For babies:

  • Babies in the elective induction group had a lower composite adverse outcome rate (1.7% vs. 3.1%). For babies, this combined outcome included perinatal death, Apgar score <7 at five minutes, arterial pH <7.05, meconium aspiration syndrome, nerve injury, brain bleeds, and admission to a newborn intensive care unit. It was mostly the lower rate of Apgar score <7 at five minutes that contributed to the lower combined adverse outcome with the elective induction group (1.2% with elective induction versus 2.6% with expectant management). The authors note that there was no difference in rates of Apgar score <4 at 5 minutes; however, the combined outcome was still significantly lower in the elective induction group if using Apgar score <4 at 5 min. and excluding congenital health problems. 
  • One stillbirth occurred in the elective induction group at 40+6 (before the mother was induced) and two stillbirths occurred in the expectant management group (while the mothers were waiting for labor at 41+3 and 41+4); there were no newborn deaths in either group. There was no protocol for fetal surveillance, it varied by local guidelines, but fetal monitoring and assessment of amniotic fluid levels was typically performed between 41-42 weeks. 

So, the INDEX trial found that elective induction at 41 weeks resulted in less overall bad outcomes for babies compared to waiting for labor until 42 weeks. However, the absolute risk of a bad outcome outcome (perinatal death, intensive care admission, Apgar score <4 at five minutes) was low in both groups (1.7% vs. 3.1%). 

The SWEPIS trial from Sweden

The SWEdish Post-term Induction Study (SWEPIS) garnered a lot of media attention with headlines like “Post-term pregnancy research cancelled after 6 babies die.” Indeed, the researchers planned to enroll 10,000 mothers from multiple centers across Sweden but ended up stopping the study early (with about 1,380 people in each group) after their Data Safety and Monitoring Board found a significant difference in perinatal death between the groups (Wennerholm et al. 2019).

In Sweden, just like in the Netherlands, it is not common to induce labor before 42 weeks with uncomplicated pregnancies. This study set out to compare elective induction at 41 weeks +0-2 days versus expectant management and induction at 42 weeks+0-1 days (if still no labor). From 2015 to 2018, researchers enrolled healthy mothers with single, head-down babies. Gestation age had to be estimated with 1st or 2nd trimester ultrasound. They excluded people with a prior Cesarean, diabetes, low fluid levels, high blood pressure disorders, small for gestational age babies, or known fetal malformations. They have a low stillbirth rate in Sweden, so they planned to enroll a lot of people (10,000), but they ended up not needing nearly that many people to see a difference in perinatal outcomes between groups.

A big strength of the SWEPIS trial is that they defined an induction protocol to use with the people assigned to elective induction and those assigned to expectant management (that ended up getting induced for medical reasons or because the mother reached 42 weeks of pregnancy). If the mother’s cervix was ripe, they broke her water and gave her synthetic oxytocin as needed; if the mother’s cervix was not ripe or the baby’s head not engaged, they used any of the following: mechanical methods, misoprostol, prostaglandins, and used synthetic oxytocin if the mother’s cervix was ripened first.

In the elective induction group, 14% of the participants had spontaneous labor before their induction and 86% were induced. In the expectant management group, 67% of the participants went into labor spontaneously before their planned induction and 33% were induced. Similar to the INDEX trial, the median reduction in length of pregnancy between groups was very slim (only 3 days difference).

What did the SWEPIS trial find?

For babies:

  • The study was stopped after five stillbirths and one early newborn death occurred in the expectant management group; no deaths occurred in the elective induction group. All five stillbirths occurred between 41 weeks 2 days, and 41 weeks 6 days. The newborn died four days after birth due to multiple organ failure. All perinatal deaths were to first-time mothers, which suggests that post-term induction may be especially beneficial for first-time mothers. They found that only 230 inductions at 41 weeks could prevent one perinatal death (a lower number than previously thought).
  • There was no difference in the composite perinatal outcome (2.2% to 2.4% in both groups). This combined outcome included perinatal death, Apgar score <7 at 5 min., pH less than 7, brain bleeds, brain injury from low oxygen, convulsions, meconium aspiration syndrome, ventilation after birth, or nerve injury. However, there was a significant difference in perinatal death alone.
  • The elective induction group babies were less likely to be admitted to intensive care (4% vs. 5.9%), they had less jaundice (1.2% vs. 2.3%), and fewer of them were big babies (4.9% vs. 8.3%).

For mothers:

  • There was no difference in Cesarean rates (10-11% both groups).
  • More mothers in the elective induction group had inflammation of the inner lining of the uterus, called endometritis (1.3% vs. 0.4%).
  • More mothers in the expectant management group developed high blood pressure disorders at the end of pregnancy (3% vs. 1.4%).
  • Qualitative data found that people who waited for labor had negative thoughts, felt in “limbo.”

It’s important to point out that fetal surveillance in this study was done per local guidelines—there was no protocol. None of the six deaths occurred in the Stockholm region of Sweden, where they do additional fetal surveillance. The mothers recruited in the Stockholm region (about half the people in the study) had ultrasound measurement of amniotic fluid volume and abdominal diameter at 41 weeks, whereas such assessments were not regularly performed at the other centers. This means that the results of the study may not apply equally to mothers receiving fetal surveillance at the end of pregnancy. 

What about people who are planning a VBAC?

Many people who are planning a VBAC are told they must go into labor by 39 weeks (or 40 or 41 weeks) or they will be required to have a repeat Cesarean or induction.

Research has shown that only about 10% of people who reach term will give birth by 39 weeks (Smith 2001; Jukic et al. 2013). So, if a hospital or physician mandates repeat Cesareans for people who have not gone into labor by 39 weeks, this means that 90% of people planning a VBAC with that hospital or physician will not be able to have a spontaneous VBAC. Also, some hospitals and providers will not provide inductions with VBACs, which means some people who reach the required deadline will only have one option– repeat Cesarean.

There is actually no evidence supporting a hard-stop “must-give-birth-by-39-weeks” rule for people planning a VBAC.

In 2015, researchers looked at 12,676 people who were electively induced at 39 weeks for a VBAC, or had expectant management for a VBAC (Palatnik & Grobman 2015).

Elective induction at 39 weeks was associated with a higher chance of VBAC compared to expectant management (73.8% vs. 60-62%), but there was also a higher rate of uterine rupture in the elective induction group (1.4% vs. 0.4-0.6%).

For people who chose not to be induced, the risk of uterine rupture was fairly steady at 39 weeks (0.5% uterine rupture rate), to 40 weeks (0.6%), to 41 weeks (0.4%).

The first large meta-analysis to specifically look at the link between weeks of pregnancy and likelihood of VBAC success was published in 2019 (Wu et al. 2019). It included 94 observational studies with nearly 240,000 people planning a VBAC. Interestingly, they found that gestational week at birth was not linked to VBAC success— whether someone gave birth at 37 weeks, 39 weeks, or 41 weeks—it didn’t make a difference to whether someone had a VBAC or a repeat Cesarean.

Is it safe for someone to wait for labor to begin on its own, if that is what they prefer? How long is it safe to wait?

When pregnant people go past their estimated due dates, it is appropriate for them and their care provider to discuss the benefits and risks of elective induction and expectant management.

Most research articles and guidelines say that because there are benefits and risks to both options, the pregnant person’s values, goals, and preferences should play a part in the decision-making process.

Ultimately, after receiving accurate, evidence-based information and guidance from their health care provider, pregnant people have the right to decide whether they prefer to induce labor, or wait for spontaneous labor with appropriate fetal monitoring.

Are there any benefits to going past your due date?

One of the major benefits of going past your due date and awaiting the spontaneous start of labor is the hormonal benefit of experiencing spontaneous labor. In her book Hormonal Physiology of Childbearing (free full text available here), Dr. Sarah Buckley reviewed the research on the hormonal benefits of spontaneous labor.

Based on the available evidence, Dr. Buckley concluded that:

“Overall, consistent and coherent evidence from physiologic understandings and human and animal studies finds that that the innate, hormonal physiology of mothers and babies—when promoted, supported, and protected—has significant benefits for both in childbearing, and likely into the future, by optimizing labor and birth, newborn transitions, breastfeeding, maternal adaptations, and maternal-infant attachment” (Executive Summary, page 9).

Another benefit of going past your due date and experiencing spontaneous labor is that you can avoid the potential risks of a medical induction, which may include experiencing a failed induction (possibly leading to a Cesarean), uterine hypercontractility (uterine contractions that are too close together and may decrease blood flow to the baby), and adverse effects of other interventions that often occur with an induction, such as epidural anesthesia and continuous fetal monitoring (NICE Guidelines, 2008).

Although anecdotally it has been said that later term and post-term babies have an easier time with breastfeeding, we were not able to find any research on that subject.

Induction at 41 weeks

Pros

  • Lower risk of stillbirth, especially among those with risk factors for stillbirth such as being pregnant with your first baby. The absolute risk of stillbirth is (Muglu et al. 2019):
    • 4 out of 10,000 pregnancies at 39 weeks
    • 7 out of 10,000 pregnancies at 40 weeks
    • 17 out of 10,000 pregnancies at 41 weeks
    • 32 out of 10,000 pregnancies at 42 weeks
  • Lower risk of baby receiving intensive care unit admission
  • Lower risk of baby having jaundice
  • Lower risk of having a big baby
  • Lower risk of Cesarean, may depend on practice setting and could be that risk only increases if an induction becomes necessary for medical reasons
  • Lower risk of a mother developing a high blood pressure disorder at the end of pregnancy
  • Convenience, end an uncomfortable pregnancy

Cons

  • Medically induced contractions (↑pain)
  • Miss the hormonal benefits of spontaneous labor
  • Potential uterine tachysystole (more than 5 contractions in 10 minutes, averaged over a 30-minute window; it can lead to a possible decrease in oxygen to the baby and fetal heart rate changes)
  • Potential for failed induction leading to a Cesarean
  • Potential for medicalization of birth and cascade of interventions (e.g., continuous fetal monitoring)
  • Increased risk of mother getting inflammation of the inner lining of the uterus (endometritis)

How should people and their care providers talk about the risk of stillbirth?

It can be difficult for health care providers and expectant parents to discuss the risk of stillbirth. Research on health care decision-making suggests that one of the best ways to frame the risk of stillbirth is to use the following techniques (Perneger & Agoritsas 2011; Fagerlin et al. 2011).

  • Present risks in actual or “absolute” numbers (as opposed to relative risk)
  • Talk about both potential gains and losses
  • Offer a visual if possible
  • Focus on the absolute difference between two risks

So, in a real life situation, this might look like:

“At 41 weeks, out of 10,000 pregnant people, about 17 will have a stillbirth. This means 9,983 won’t have a stillbirth.

In comparison, at 42 weeks, out of 10,000 pregnant people, about 32 will have a stillbirth. This means 9,968 won’t have a stillbirth. Here is a picture to help give you an idea of what this means.

So an extra 15 people out of 10,000 might avoid a stillbirth by being induced at 41 weeks. For the other 9,985 women, it won’t make a difference.”

Then circle/highlight the additional 15 to show the difference.

 

What do the guidelines say?

  • In an extensive evidence review from the U.S. Agency for Healthcare Research and Quality, researchers found that elective induction at 41 weeks decreases the risk of Cesarean and meconium aspiration syndrome, but that we’re not sure how this evidence translates into real-life settings. To read the free full text of this e-book, click here. (Caughey et al. 2009)
  • The Ontario Midwives Association has a really comprehensive, easy-to-understand set of guidelines. To download the free PDF, click here.
  • To download the Society of Obstetricians and Gynaecologists of Canada guidelines (Canada), click here.
  • In 2019, ACOG reaffirmed their 2014 recommendations on post-term pregnancy. Although their guidelines are not freely available to the public, ACOG recommends that induction of labor should take place between 42 weeks 0 days and 42 weeks 6 days, and that induction at 41 weeks can also be considered. If a person planning a VBAC goes post-term, this does not mean they have to have a repeat Cesarean. Also, ACOG released new practice guidelines that address the ARRIVE trial findings (ACOG/SMFM, 2018). They concluded that it is reasonable to offer elective induction to low-risk, first-time mothers at 39 weeks of pregnancy. However, they urge care providers to first consider three important factors: the values and preferences of the pregnant woman, the staffing and facility resources available (to assist longer labors), and the protocol for “failed” induction. Specifically, as long as there are no complications, early labor can last 24 hours or more and oxytocin can be given for 12 to 18 hours after breaking the mother’s water before the induction is considered a failure.
  • The American College of Nurse-Midwives (ACNM) also released a press statement in response to the ARRIVE trial saying that they continue to promote normal healthy physiologic birth and a woman’s right to make decisions during pregnancy (ACNM, 2018). They expressed concern that many women may not desire elective induction and proposed that costs might be better spent on less invasive approaches to reduce Cesareans, such as continuous labor support from a doula.

The bottom line

The whole topic of due dates and induction for due dates can be very confusing. There is a lot of information to digest. We feel that the most important points can be summarized like this:

Due dates

  • The traditional way of calculating the estimated due date (40 weeks after the last menstrual period) is not evidence-based. Instead, it is more accurate to give people a range of time that they will probably give birth:
    • About half of first-time mothers will give birth by 40 weeks and 5 days after the LMP, with the other half giving birth after that time point
    • About half of mothers who have given birth before will give birth by 40 weeks and 3 days after the LMP, with the other half giving birth after that
    • An ultrasound before 20 weeks is usually more accurate than using the last menstrual period, and the accuracy of an ultrasound is highest if it is done between 11 and 14 weeks.

Elective induction versus waiting for labor

  • There is no clear right or wrong path with regard to elective induction between 39-41 weeks or continuing to wait for labor.
  • People can talk with their care providers about the pros and cons of waiting for spontaneous labor or elective induction (see Pros/Cons lists above). This conversation should take into account the mother’s preferences, goals, values, personal birth history, chances of a successful induction (how “ripe” the cervix is, also known as the Bishop score”), and alternatives.

Final thoughts

It’s important to note that none of the research evidence looked closely at birthing people’s experiences or preferences. These non-medical factors are very real when it comes to individual decision-making. For example, it may be very important to a healthy first-time mother who very much wants an unmedicated birth to have a spontaneous labor, while it may be very important to a mother who has experienced miscarriages or stillbirth in the past to lower the absolute risk of stillbirth by any means necessary.  All of these experiences and preferences are valid.

References:

Acknowledgments:

I would like to extend my deepest gratitude to Sonja Billes, PhD, and Robert Modugno, MD, MBA, FACOG, who traveled to Lexington, Kentucky in 2014 to help me conduct the first extensive literature search for this article. I would also like to thank my expert reviewers for an earlier version of this article—Shannon J. Voogt, MD, Board-Certified in Family Medicine; Tara Elrod, CDM of Integrated Women’s Wellness in Alaska; Heather Thompson, MS, PhD, Research Director of Mountain Midwifery Center, and Deputy Director of Elephant Circle; Mimi Niles, CNM, MSN, MPH, and PhD student; and Cynthia B. Flynn, CNM, PhD, FACNM, Past President of the American Association of Birth Centers, expert midwife at www.pregnancy.org and founder of Birth Center Consulting.

I would also like to thank Cristen Pascucci and Sharon Muza CD(DONA), BDT(DONA), LCCE, FACCE for their medical editing assistance.

Photo credit to: Birth Becomes Her for the featured image of this article.  

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