Calculate your due date

This article was written by Rebecca Dekker, PhD, RN, APRN

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 C-section? At which point do the benefits of being electively induced outweigh the risks? Do women’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 women induced for going past their estimated “due date?”

Inductions for non-medical reasons have been on the rise in the U.S. Increasingly, more women 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. 2014). The researchers asked women to select the reasons that they were induced. What was the single most common reason for labor induction? Out of all women who were 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.”

Why is there so much controversy about elective induction?

For many years, the common belief was that elective (not medically indicated) inductions doubled the C-section 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. 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 C-section rates—were flawed. In the earlier studies, elective induction was compared only to spontaneous labor: women who were electively induced vs. women who went into spontaneous labor.  Excluded from these two groups are women who were not electively induced, but chose to wait for labor and then ended up having medically indicated inductions later on (and, thus, a higher rate of C-sections). For an example of this earlier flawed research, see this article by Yeast et al. 1999.

New researchers pointed out that we need to compare women who have elective inductions with the whole group of women who choose to wait for spontaneous labor—whether or not they actually do have spontaneous labor. This is a subtle difference, but an important one, because not all women who choose to wait will actually have a spontaneous labor; some of them will develop complications that lead to an induction and increase their risk for C-section.  The researchers argued that the comparison group must include that group of women. This graphic shows how you would look at the two groups: the elective induction group vs. the entire group of women 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 (see this PowerPoint by Dr. Robert Silver for more information).

Enter the ARRIVE study.

Funded by the U.S. National Institutes of Health, the doctors in charge of the ARRIVE study (A Randomized Trial of Induction Versus Expectant Management) are currently enrolling 6,000 first-time moms from across the U.S. These women are being randomly assigned to elective induction at 39 weeks OR waiting for labor to start on its own (expectant management), up until 41 weeks. According to the informed consent document for the study, “The goal of the study is to find out whether coming to the hospital and having your labor started with medicine (induced) at 39 weeks of pregnancy can improve the baby’s health at birth when compared with waiting for labor to start on its own.

Vanessa was planning on another natural birth with her 3rd child. At 39 weeks she elected for an induction due to severe, chronic sciatica pain. She was surprised by the intensity of the Pitocin-induced contractions, and needed an epidural. Vanessa ended up with an emergency Cesarean when her baby (who was moving constantly during the intense contractions) flipped breech/ transverse during the induction.

The study brochure (used to recruit women into the study) says that “During labor induction, the same types of complications that can arise during spontaneous labor can occur.” Unfortunately, this statement is not quite true, because risks of inductions include hyper-stimulation of the uterus (where the uterus contracts too frequently, decreasing blood flow to the baby), the use of extra interventions such as continuous fetal monitoring and the need for additional pain relief, and a failed induction leading to a Cesarean (NICE Guidelines, 2008). Although the researchers are looking at the benefits and risks of elective induction at 39 weeks—including Cesarean rates, serious infant health problems, hospital costs, and patient satisfaction—they are not looking at the bigger implications of their goal. Their goal is to find out that when all women give birth by 39 weeks, there are better outcomes than letting women wait for spontaneous labor. But since only 10% of women go into labor on their own by 39 weeks (Smith 2001; Jukic et al. 2013), what would happen if labor and delivery units all over the world were filled with women having elective inductions at 39 weeks? Would there be any unintended effects? What might happen to a woman 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 women being electively induced at 39 weeks?  Some people may argue that this elective induction epidemic is already happening… so we need research in order to learn the pros and cons of elective induction at 39 weeks! However, one thing that the ARRIVE study won’t tell us is whether or not induction at 39 weeks can decrease the risk of stillbirth or newborn death. In earlier research, it took a sample size of at least 7,000 women to tell whether elective induction at 41-42 weeks decreases the combined risk of stillbirth and newborn death (Hussain et al. 2011; Gulmezoglu et al. 2012). Since stillbirths and newborn deaths are even rarer at 39 and 40 weeks, the ARRIVE study (with 6,000 women) is simply too small to tell if elective induction will have an effect on this outcome.

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). For example, if your last menstrual period was on April 4, 2015, you would add seven days (April 11, 2015) and subtract 3 months = an estimated due date of January 11, 2016. Another way to look at it is to say that your EDD is 40 weeks after the first day of your last period.

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.

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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 2010 Cochrane review, researchers combined the results from 7 randomized clinical trials that compared routine early ultrasound to a policy of not routinely offering ultrasound. The researchers found that women 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 women 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 2014). LMP is less accurate because it can have these problems:

  • Women can have irregular menstrual cycles, or cycles that are not 28 days
  • Women may be uncertain about the date of their LMP
  • Many women do not ovulate on the 14th day of their cycle
  • The embryo may take longer to implant in the uterus for some women
  • 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 recent 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 women 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 women (26%) reported that their care provider changed their estimated due date based on a late pregnancy ultrasound. For 66% of the women, the estimated due date was moved up to an earlier date, while for 34% of women the date was moved back to a later date (Declercq et al. 2014).

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 (2014). They suggest that the due date should only be changed after a third trimester pregnancy ultrasound if 1) it is the woman’s first ultrasound, and 2) it is more than 21 days different than the due date suggested by the LMP (ACOG 2014).

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 women 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 women, and measuring the time from ovulation (or the last menstrual period, or an ultrasound) until the date the women gave birth—and calculating the average. However, this method is wrong and does not give us accurate results.

Alicia, a certified doula, birthed her second daughter exactly on her EDD. At her 40 week appointment with her OB she was informed that she was two centimeters dilated and they talked about waiting for induction until 42 weeks. That same afternoon labor began! Her daughter was born the next morning. She described her birth as “a beautiful, happy, identity-changing, empowering experience”.

Why is this method wrong?

This method does not work because many women are induced when they reach 39, 40, 41, or 42 weeks. If you do include these induced women in your average, then you are including women 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 woman who was induced at 42 weeks from your study, then you are ignoring a pregnancy that was induced because she went longer—and by excluding her, 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 women in your study, and still get an accurate picture of how long it takes the average woman 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 enrolled, and they all gave birth between the years 1982 and 1985. However, this was also an important study, because researchers followed these women 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 women ovulated, conceived, and even when their pregnancies implanted!

Sigrid, a newborn nurse and lactation consultant at a busy urban hospital, was planning a second home birth VBAC with her fourth baby. When she reached 42 weeks, she risked out of a home birth and began planning for a hospital birth. When labor finally started on its own at 43 weeks, her baby was born too quickly to make it to the hospital, and she ended up with an accidental unassisted home birth after Cesarean. Sigrid said that her daughter “took FOREVER to decide to be born, but when she wanted out, she meant business!“

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 woman 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 woman 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 these women 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 women 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 women give birth. About half of all women 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 women are 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.

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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 had 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 genes. 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 women and/or fetuses have a genetically decreased tendency to start labor. Other factors that may make your pregnancy more likely to go longer include:

With her first pregnancy, Shannon did not go into labor on her own by 42 weeks, and so she was induced with Cytotec. Her drug-induced contractions were very intense– she had contractions that lasted 60-90 seconds every 1 and a half minutes for 10 hours. Shannon gave birth vaginally to a healthy 7 lb, 8 oz baby, although there was some meconium aspiration. Two years later, Shannon gave birth again spontaneously at 41 weeks and 2 days (after membrane stripping), and her sister had a post-term pregnancy and went into labor spontaneously at 42 weeks 5 days.  With her first pregnancy, Shannon did not go into labor on her own by 42 weeks, and so she was induced with Cytotec. Her drug-induced contractions were very intense– she had contractions that lasted 60-90 seconds every 1 and a half minutes for 10 hours. Shannon gave birth vaginally to a healthy 7 lb, 8 oz baby, although there was some meconium aspiration. Two years later, Shannon gave birth again spontaneously at 41 weeks and 2 days (after membrane stripping), and her sister had a post-term pregnancy and went into labor spontaneously at 42 weeks 5 days.

Published April 15, 2015, © Evidence Based Birth®, All Rights Reserved.