One of the first things you do when you find out you are pregnant is to 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 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 estimates when your baby will arrive, it is not an exact science. 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 examines how to calculate your due date, the accuracy of these calculations, and what this means for your pregnancy. 

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Your Due Date

Your due date is the date that your baby should theoretically be born. This date lands on the start of your 40th week of pregnancy. The American College of Obstetricians and Gynecologists classifies pregnancy terms into four separate categories based on how old your baby is at birth.

  • 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

Let’s examine the evidence on how long pregnancy lasts. One study found it was most accurate to add 282 days to the last menstrual period. This study included mothers who had a labor induction. Another study found that the average length of pregnancy was 283 days. Researchers found no difference in pregnancy length between having a boy or a girl. There was a difference in women who already had at least one baby in that the median gestational age at birth was two days shorter. This study did exclude women who were induced, so it looked at the duration of pregnancy without this intervention.

Remember, these numbers are averages. I could not find a statistic on how many babies are born on their due date, and most estimates put that number between 3-5%. According to 2020 CDC data:

  • 10.09% of babies were born pre term (before 37 weeks) 
  • 27.78% of babies were born early term (37-38 weeks 6 days) 
  • 57 % of babies were born full term (39 weeks-40 weeks 6 days) 
  • 5.13% of babies were born late and post term (41 weeks and beyond) 

Your due date is only an estimate. It is a helpful data point to organize your prenatal care and planning, but it is not a deadline.

How Your Due Date Drives Your Prenatal Care 

Your care provider organizes your entire prenatal around your due date. This date drives the timing of your prenatal appointments. These start in week eight and are 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 see your care provider weekly. If you go past 40 weeks, your appointments can be every few days.

The timing of when you take some prenatal tests is 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 induction is even more date sensitive. You can see the whole calendar of your prenatal care revolves around 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 a birth class, and the timing of your baby shower. There are windows of time 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 to 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 earlier in your pregnancy 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 two 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 on 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 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. Going past your date can bring a whole new set of emotions about your baby arriving. Depending on where you plan on having your baby and your care provider, you may have some limitations on how far past your due date you go. Few care providers are comfortable letting you go past 42 weeks due to an increased stillbirth rate.

Naegele’s Rule to Calculate Your Due Date

The calculation most commonly used today to calculate your due date is Naegele’s rule. Herman Boerhaave, a professor of botany and medicine in the Netherlands, first developed the formula to calculate the due date in the 1700s. There is some speculation that Boerhaave based this formula on observations found in the Bible that human gestation lasted ten lunar months, or 280 days. A German obstetrician in the late 1700s named Franz Karl Naegele first put this formula into practice. 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 seven days and go forward nine 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 is difficult to know when conception occurs. Sperm can survive for up to seven days before fertilizing an egg. The time from ovulation to implantation can be as long as 11 days.

This formula also 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 two days. If you are trying to conceive, it can be helpful to track your cycles with an app or a note, so you have an accurate recording of when you get your period.

Pregnancy Wheel 

In the past, care providers used a pregnancy wheel to calculate your due date based on Naegele’s rule. These pregnancy wheels usually have a lot of information, like when some tests would take place, like an amniocentesis or CVS test. It could include your baby’s estimated weight and length throughout your pregnancy. While they were helpful, they were also marketing tools often distributed by pharmaceutical companies.

The American College of Obstetricians and Gynecologists has an app that has replaced pregnancy wheels. The app can estimate the due date based on the last menstrual period (using Naegele’s rule), ultrasound, or assisted reproductive technology. It makes sense that ACOG would want to standardize this across all OB/GYNs, and 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). 

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 tiny, the measurement taken would be the diameter of the sac. This has an accuracy of 4-11 days. 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. Your care provider can take some other measurements later in your pregnancy that are less accurate. This includes head circumference, abdominal circumference, and femur length. The accuracy will also depend on the technician’s skill in operating the ultrasound. Ultrasound measurements for dating pregnancy are more accurate earlier on in your pregnancy. 

Are Ultrasounds Better than Calculating from the Last Menstrual Period? 

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 one study, menstrual dating underestimates the ultrasound-based due date by an average of two to three days. The researchers state that based on the available research, ultrasound-derived dates are 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 last menstrual period. This study showed when ultrasound was used instead of the last menstrual period, the number of post-term pregnancies decreased from 10.3% to 2.7%. The more women that delivered within 42 weeks, the more accurate it is believed the calculation is. 

According to 2020 CDC data, 77.7% of women begin prenatal care in the first trimester. This means that nearly one in four women would not have access to ultrasound measurements for a due date from the first trimester and would be estimating their due date from their last period or a later ultrasound. 

The American College of Obstetricians and Gynecologists 

A committee opinion from ACOG 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. Subsequent changes to the EDD should be reserved for rare circumstances.

Reconciling Last Menstrual Period 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 9 weeks of gestation differs by more than five days from the last menstrual period, the estimated due date should be changed to correspond with the ultrasound dating. Ultrasound dating between 9 and 14 weeks of gestation needs to differ by more than seven days from the last menstrual period dating to change the due date according to the ultrasound. In the second and third trimesters, the accuracy of ultrasound dating declines, so there would need to be an even more significant difference in the ultrasound date and your last menstrual period date for your doctor or midwife to change the date to the ultrasound date. 

Your care provider is 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. See this episode for an examination of the evidence on ultrasounds.

How Will We Calculate Due Dates in the Future? 

In one study, researchers suggested an alternative to using Naegele’s rule. They argue a more evidence-based approach might be to assign a range of due dates (perhaps the interquartile range, in which half of the women will deliver) or to describe the due date as a median (before which half of the 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 current 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 are you supposed to do with all of this information? 

This article had a lot of data, and there are some key takeaways to keep in mind. Your due date is a valuable data point to organize your prenatal care and planning. It is only an estimate, not exact date or deadline.

If you are trying to get pregnant, track your cycles. You can also track when you are having sex to try and pinpoint a conception date. You can use an app to track these dates, make a note, or mark it on your calendar. This data will be helpful when you do get pregnant.

If you had no idea when your last menstrual period was, if you had irregular periods, or if you didn’t find out you were pregnant until later on, don’t stress. Your doctor or midwife can obtain a relatively accurate due date with an ultrasound.

Your due date being exact may not seem significant early on. As you reach the last weeks of pregnancy, your due date has a considerable influence on interventions like inducing labor.

Talking to Your Doctor or Midwife 

As always, your doctor or midwife is your partner in your prenatal care. Please discuss any questions or concerns about your due date with your care provider.

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