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One of the first things you do when you find out you are pregnant determine your due date. This date rules everything about your pregnancy from your prenatal care to the timing of your baby shower. This is also the most common question you will get from friends, family, and strangers. As you get close to your due date there can be a lot of anxiety, especially if your due date comes and goes and you are still pregnant. This date is an estimate of when your baby will arrive, it is not an exact science, and it is certainly not a deadline. The calculation we use today is hundreds of years old and the evidence doesn’t 100% agree with the calculation. This episode dives into how your due date is calculated, the accuracy of these calculations, the evidence of how long pregnancy lasts, and what this means for your pregnancy.

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How Your Due Date Drives Your Prenatal Care

Your entire prenatal care is based on your due date. This date drives the timing of your prenatal appointments. These start out monthly through week 28. Beginning in week 29, which is the start of the third trimester, you will go every other week until 36 weeks. From week 37 you are seeing your care provider on a weekly basis. If you go past 40 weeks your appointments can be every few days.

The timing of when tests are administered are also driven by your due date. This includes everything from a CVS test to your anatomy scan ultrasound. As you approach your due date timing of interventions like an induction are even more date sensitive. You can see the whole calendar of your prenatal care is based on your due date.

Planning Your Life Around Your Due Date

In addition to timing appointments, you will also be planning your life around your due date. This includes big projects like planning for your nursery, deciding when to take birth classes and the timing of your baby shower. There are windows that are ideal for planning these events and being off by a week or two shouldn’t make a huge difference. When it comes to planning your maternity leave days can make a big difference. If you have family coming into town to help out when your baby is born it can be tough to pinpoint an ideal date. If you are a planner, it is a challenge not to know exactly when your baby will arrive.

Fielding Questions on Your Due Date

Everyone asks when your baby is due. This may be a simple question early on and can get more challenging as you get closer to your due date. Hearing someone say, “Wow you must be ready to go into labor anytime!” when you have 2 months left can be awkward. Sharing your due date with strangers usually means you will get their opinion about that time of year. With my daughter, who was due December 26th, the most common reply was “Oh a Christmas baby!” which by the tone I could usually tell whether they thought that was a positive or negative thing. You can be specific about the date or give people a ballpark depending on how much you want to share and how much of their opinion you want back.

Anxiety About Your Due Date

It is common to be nervous as you approach your due date. The general consensus is when you hit 37 weeks you could go into labor at any time. That leaves a five-week window of waking up every day wondering if this is the day you get to meet your baby.

If you go past your date that can bring a whole new set of emotions about your baby arriving. Depending on where you are planning on having your baby and your care provider you may have some limitations as to how far past your due date you go. Few care providers are comfortable letting you go past 42 weeks due to an increased rate of stillbirth. 

Defining a Term Pregnancy

In the past, a baby born between 37-42 weeks was concerned born at term. In 2013 the American College of Obstetricians and Gynecologists revised their classifications for pregnancy terms into four separate categories.

  • Early term 37-38 weeks 6 days
  • Full-term 39 weeks-40 weeks 6 days
  • Late-term 41 weeks and 41 weeks 6 days
  • Post-term After 42 weeks

The Evidence on Pregnancy Duration

What does the evidence say about how long pregnancy lasts?

One study found it was most accurate to add 282 days to the LMP, rather than the 280 suggested by Naegele’s rule (explained later). This study included mothers who had a labor induction. Another study found that the average length of pregnancy was 283 days, rather than the 280 used in Naegele’s rule. There was no difference found for having a boy or girl, the gestation of one was not shorter than the other. There was a difference if it was a woman who already had at least one baby, in that the median gestational age at birth was 2 days shorter. This study did exclude women who were induced, so it was looking at the duration of pregnancy without this intervention.

Remember these numbers are averages. Many expecting mothers are having babies past this date, it is not a deadline.

I was not able to find a statistic on how many babies are born on their due date and most estimates put that number between 3-5%. According to 2018 CDC data:

  • 10.02% of babies were born preterm (before 37 weeks)
  • 26.53% of babies were born early term (37-38 weeks 6 days)
  • 57.24% of babies were born full-term (39 weeks-40 weeks 6 days)
  • 6.2% of babies were born late and post-term (41 weeks and beyond)

Keep in mind that in the United States the cesarean rate is 31.9% which would have some effect on these figures.

The History of Your Due Date Calculation

The calculation most commonly used today to calculate your due date is based goes back to the 1700s.

Naegele’s rule

Herman Boerhaave, a professor of botany and medicine in the Netherlands, first developed the formula to calculate due date from the last menses. There is some speculation that Boerhaave based this formula on observations found in the Bible that human gestation lasted 10 lunar months, or 280 days. This formula was first put into practice by a German obstetrician in the late 1700s, named Franz Karl Naegele. Today we know this formula as Naegele’s Rule.

The easiest way to calculate your due date using Naegele’s rule is to take the first day of your last period, add 7 days and go forward 9 months. This works out to be roughly 280 days from the start of your last menstrual period.

Your due date = First day of your last period + 7 days + 9 months

Assumptions and Limitations of Naegele’s Rule

Naegele’s rule operates on several assumptions. It assumes a 28-day cycle and ovulation on day 14. This formula doesn’t account for leap years or different days each month. It assumes you ovulated on day 14 of your 28-day cycle. Sperm can survive for up to 7 days before fertilizing an egg. It is really difficult to know when conception occurs. The time from ovulation to implantation can be as long as 11 days.

This formula assumes you know the exact date you started your last menstrual period. In a study that looked at how accurately women recalled the date of the start of their last period, 56% were accurate, 74% were within one day, and 81% were within 2 days. If you are trying to conceive it can be helpful to track your cycles with an app.

You can see that there are quite a few assumptions that leave a lot of room for error with this calculation.

Pregnancy Wheel

In the past care provider used a pregnancy wheel to calculate your due date at your first appointment. These handy wheels are often manufactured and distributed by pharmaceutical companies, they are a great marketing tool. These are all based on Naegele’s rule and I have read that there can be discrepancies in due dates depending on the wheel used. Remember these are a marketing tool, not a resource put out by an organization like ACOG.

Pregnancy wheels usually have a lot of information on them like when some tests would take place, like an amniocentesis or CVS test. It could include the estimated weight and length your baby should be throughout your pregnancy.

The American College of Obstetricians and Gynecologists has an app that has replaced pregnancy wheels. The app can estimate the due date based on LMP, ultrasound, or ART. It makes sense that ACOG would want to standardize this across all OB/GYNs. They state the app is based on joint recommendations from ACOG, the American Institute of Ultrasound in Medicine (AIUM) and the Society for Maternal-Fetal Medicine (SMFM). When calculating from LMP the app uses Naegele’s rule. 

Ultrasound dating

As ultrasound technology has improved over the last few decades ultrasound dating has become routinely used. An ultrasound tech or your care provider can use an ultrasound to take measurements that can help determine gestational age. Earliest on, when your baby is really tiny the measurement taken would be the diameter of the sac. This has an accuracy of 4-11 days, meaning the date predicted will be within 4-11 days of the actual date.

The most accurate measurement is to measure your baby’s crown to rump length. This is the length from the crown of their head to their bottom. This has an accuracy of 3-8 days. There are some other measurements that can be taken later in your pregnancy that are less accurate. This includes head circumference, abdominal circumference, and femur length. The accuracy will also depend on the skill of the technician operating the ultrasound. Ultrasound measurements for dating pregnancy are more accurate earlier on in your pregnancy.

Are Ultrasounds Better than LMP?

The general consensus is that the current gold standard for estimating your due date is by ultrasound. No research that I reviewed found dating from the last menstrual period was more accurate than ultrasound dating. According to a study, menstrual dating underestimates the ultrasound-based due date by an average of 2 to 3 days. The researchers state that based on the available research, the use of ultrasound derived dates is the best method to determine gestational age for clinical use.

Quite a bit of evidence does show ultrasounds to be more accurate at dating a pregnancy than the LMP. This study showed when ultrasound was used instead of certain LMP, the number of post-term pregnancies decreased from 10.3% to 2.7%. Basically the more women that delivered within 42 weeks the more accurate it is believed the calculation is.

According to CDC data, 77% of women begin prenatal care in the first trimester. This means that nearly one in four women would not have access to ultrasound measurements in the first trimester and would be estimating due date from their last period or a later ultrasound.

The American College of Obstetricians and Gynecologists

In a Committee Opinion from ACOG, they stated: Accurate dating of pregnancy is important to improve outcomes and is a research and public health imperative. As soon as data from the last menstrual period, the first accurate ultrasound examination, or both are obtained, the gestational age and the estimated due date (EDD) should be determined, discussed with the patient, and documented clearly in the medical record.

Reconciling LMP and Ultrasound Dates

What happens when your due date from your last menstrual period and your ultrasound due date don’t match up? According to ACOG if ultrasound dating before 14 weeks of gestation differs by more than 7 days from LMP dating, the estimated due date should be changed to correspond with the ultrasound dating. If the due date according to your LMP and ultrasound are off by less than 7 days you would stick with your LMP due date.

In the second and third trimesters, the accuracy of ultrasound dating declines so there would need to be an even bigger difference in the ultrasound date and your LMP date for your doctor or midwife to change the date to the ultrasound date.

Your care provider is really motivated to nail down your due date in the first trimester. If you have additional ultrasounds in the second or third trimesters, they aren’t going to change your due date according to new measurements. Also, keep in mind that not everyone chooses to have ultrasounds. Like everything, this is something you can opt-out of if you wish. Wanting to get a more accurate date, especially if you are unsure of the date of your last period, could be a reason to opt into a first-trimester ultrasound.

How Will We Calculate Due Dates in the Future?

In another interesting study researchers suggested that rather than using Naegele’s rule, a more evidence-based approach might be to assign a range of due dates (perhaps the interquartile range, in which half of women will deliver) or to describe the due date as a median (before which half of women will deliver). Clinical prediction of dates might also consider the lengths of a woman’s other pregnancies. In the data in this study, the average length of a woman’s other pregnancies was strongly related to gestational length in her study pregnancy. As more research is done and technology improves we will hopefully have more accurate ways of determining pregnancy length and calculating an expected due date in the future.

What do you do with all of this information?

This article had a lot of data and there are some key takeaways to keep in mind:

  • If you are trying to get pregnant track your cycles, you can even track when you are having sex to try and pinpoint a conception date. You can use an app to track this, or just make a note or mark on your calendar. This data will be useful when you do get pregnant.
  • Do not stress if you are unsure when your last menstrual period was, or you didn’t find out you were pregnant until later on.
  • Your due date does give you a mark for your pregnancy timeline, your prenatal care, and for you to plan. Your estimated due date is just an estimate, not an exact date, or a deadline.
  • If you would like to opt-out of interventions like induction or a cesarean try and remember that after your due date, your baby isn’t late. There is room for error in any method of calculating your due date, and not everyone has the same pregnancy length. There are numerous factors that play a part in this.

Talking to Your Doctor or Midwife

Please talk to your doctor or midwife about any questions or concerns about your due date.


Thank you to the amazing companies that have supported this episode.

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