I’m here to tell you that according to the research, there is no such thing as a “due date”. In this post, you’ll learn the duration ranges of typical pregnancies, what an estimated due date really means, and how having an incorrect due date might impact your healthcare provider's recommendations for your birth.
Reaching full term
Until recently a baby was considered to have reached full term between 37 weeks and 0 days and 41 weeks and 6 days, Anything before this range was considered pre-term, and anything after would be considered post-term.
More recently, experts created a new definition of pregnancy terms based on extensive evidence and research (Spong, 2013). You can see that the timeframe has been cut into more nuanced groups below.
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
The most accurate way to calculate your estimated due date
A Cochrane review of 11 randomized clinical trials found that people who had an early ultrasound to calculate their due date were less likely to be induced for post-term pregnancy compared to those who did not use an early ultrasound (Whitworth et al. 2015). This means that those using the Last Period Method (LPM) to calculate their estimated due date were more likely to be mislabeled as “post-term” and more likely to get an unnecessary induction.
Research has indicated that the most accurate time to estimate gestational age using ultrasound was between 11-14 weeks of pregnancy. There is a significant decline in accuracy from ultrasounds that are conducted at 20 weeks or later.
A Note on the Last Period Method
Most doctors today, when not using an ultrasound, use a form of Naegele’s rule which adds 7 days to the first day of your last period and then counts forward for 9 months. This method is not based on evidence and may even misinterpret the calculation intended by its creator.
For many reasons, the last period method or an interpretation of Naegele’s rule are less accurate than ultrasounds. Reasons include
Many people have irregular menstrual cycles that are not exactly 28 days
People may not remember the exact date of their last period
Many people ovulate on different days where this method assumes all people ovulate on the 14th day
It’s possible that the embryo takes longer to implant in the uterus for some people impacting the estimated due date
(Savitz et al., 2002; Jukic et al., 2013; ACOG, 2017)
Why I Call Due Dates “Guess Dates”
Based on the evidence, there is no such thing as an exact due date. The norm of estimating due dates at 40 weeks is not accurate. Instead, people should be told that it is normal for them to give birth during a range of dates. Using the traditional due date calculation, “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” (Evidence Based Birth).
Based on a study conducted in 2013 where the researchers knew the exact days the participants ovulated, conceived, and when the pregnancies implanted (Jukic et al., 2013), we have the following ranges for weeks for pregnancy duration (*note this is calculated using the date of ovulation):
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
Estimated due dates that are calculated using the day of ovulation are more accurate than using the last period since it does not assume ovulation at day 14.
If this study were calculating the weeks using the last period method, these would be the date ranges:
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
You can see how 10% still have not given birth by 44 weeks after their last period. This is because some participants did not ovulate on the 14th day of their period.
Why knowing the truth about due dates can impact your birth
Depending on your provider, they may recommend that you be induced once you reach a certain number of weeks according to the guess date they have calculated. If your guess date was miscalculated, you may be recommended for induction when it is not in fact medically necessary. It's important to take this into consideration when deciding whether you will elect to get induced or not.
The reason many providers recommend inductions based on due dates is because research has shown that inducing labor at 41 weeks and 0-2 days instead of waiting for labor to start on its own could help reduce the rates of poor health outcomes for the baby or stillbirth (Wennerholm, 2019). While the risk of stillbirth does increase rapidly after 41 weeks, you should make a decision based on what level of risk feels comfortable for you. Below is the risk of stillbirth by each week of gestation. Keep in mind that 17 per 10,000 = 0.0017%
39 weeks = 4 per 10,000
40 weeks = 7 per 10,000
41 weeks = 17 per 10,000
42 weeks = 32 per 10,000
Note: The risk of stillbirth is higher for those giving birth to their first baby, or are older, plus-size, have health problems, or have a fetus with growth restriction. Racism (including prejudice and institutional racism) also increases stillbirth rates.
Take some time to reflect on how your due date was calculated. Did you have your due date calculated based on an ultrasound before 20 weeks? Did you use the last period method? Did you use the last period method but knew your date of ovulation with certainty? All these factors will influence your confidence in your guess date.
I hope you walk away from this post understanding that it's normal to give birth along a much larger window than just your guess date. If you are approaching 41 weeks and your provider is recommending induction, I highly recommend reading the Evidence Based Birth article "Evidence on: Inducing for Due Dates" so you can make an informed choice.
Disclaimer: This information does not substitute for a care provider-patient relationship and should not be relied on as personal medical advice.
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Dekker, R and Bertone A. (2019). The Evidence on: Due Dates. Evidence Based Birth.
Savitz, D. A., Terry, Jr., J. W., et al. (2002). Comparison of pregnancy dating by last menstrual period, ultrasound scanning, and their combination. Am J Obstet Gynecol 187(6): 1660-1666.
Spong, C. Y. (2013). Defining “term” pregnancy: recommendations from the Defining “Term” Pregnancy Workgroup. JAMA 309(23): 2445-2446.
Wennerholm, U. B., et al. (2019). Induction of labour at 41 weeks versus expectant management and induction of labour at 42 weeks (SWEdish Post-term Induction
Study, SWEPIS): multicentre, open label, randomised, superiority trial. BMJ, 367, l6131.
Meet the Author
Clara O'Rourke (she hers),
Holistic Doula & Childbirth Educator.
I am a researcher turned total birth nerd with a passion for supporting people's birth in their full power! I guide people through their pregnancy, labor, and postpartum journey by helping them overcome fear or anxiety about birth, providing evidence-based information so they can make informed decisions, and making sure they have everything they need to birth with confidence. Birth is sacred and it should be treated that way. I am committed to providing support for birth and beyond, regardless of a person's background, sexual orientation, gender identity or expression, or relationship status. También ofrezco mis servicios en español. I wish you and your growing family the brightest of beginnings!