Are Pregnancy ‘Due Dates’ Accurate?
Two words you hear a LOT throughout pregnancy are ‘due date’. But you may wonder, much like me, if there’s an accurate way to know when exactly a baby should be born?
It’s important to understand HOW providers calculate this number, if the number is accurate, and if there are any risks for going past the ‘due date’.
Let’s dive into:
The history behind the medical model for ‘estimated due date’
Different methods of calculating the ‘estimated due date’
The accuracy of ‘due dates’
The average length of pregnancy
Risks of going past your ‘estimated due date’
Learning about ‘estimated due dates’ can help you advocate for yourself & your baby throughout your pregnancy. My whole purpose for creating this blog was to empower mothers in their decisions and the BEST way to do that is through education!
History Behind The Medical Model For ‘Estimated Due Date’
The first documented medical concept behind an ‘estimated due date’ started around 1744, when a professor named Hermann Boerhaave figured out a way to calculate pregnant women’s ‘due date’. We know women have been using the cycles of the moon to track their cycles and pregnancies for millennia, however.
Boerhaave took the records of around 100 women and estimated the due date by adding 7 days to the last menstrual cycle. He would then add 9 months to that date.
He never went into detail about whether adding those 7 days should be the FIRST day of the last menstrual cycle or the LAST day.
Carl Naegele, a professor from Germany, quoted Boerhaave’s findings and this is how ‘Naegele’s Rule’ came into existence.
However, Naegele didn’t clarify whether you should start counting from the START of the last menstrual cycle or the END of the last menstrual cycle.
Almost ALL providers still use a form of Naegele’s rule where you begin on the first day of the last menstrual period (LMP), subtract 3 months, add 7 days, and then add 1 year.
Naegele’s Rule is not based on any current evidence. This leaves a LOT of grey areas and makes you wonder why medical providers still use this method today…
Different Methods of Calculating The ‘Estimated Due Date’
There are a few different methods to calculate the ‘estimated due date’ in pregnancy. Some methods have been used for centuries & others are fairly ‘new’ still!
Naegele’s Rule: Here is Naegele’s Rule again! This method is based on a 28-day menstrual cycle & accounts for the pregnant person ovulating on the 14th day exactly (yeah…)
Ultrasound Measurement: Providers started using Ultrasound Measurement in the 1970’s. It is typically done in the first trimester.
Woods Method: Carol Wood, a nurse-midwifery professor, came up with a method to calculate the due date that takes into account individual variations in the menstrual cycle as well as the effect of a woman’s having had previous pregnancies.
Add 1 year to the first day of the last menstrual period, then
For first-time mothers, subtract 2 months and 2 weeks
For multiparas, subtract 2 months and 2.5 weeks (18 days)
Add or subtract the number of days her cycle varies from 28 days
1st-time mothers with 28-day cycles: LMP + 12 months – 2 months, 14 days= EDD (estimated due date)
Multiparas with 28-day cycles: LMP + 12 months – 2 months, 18 days = EDD
For cycles longer than 28 days: EDD + (actual length of cycle – 28 days) = EDD
For cycles shorter than 28 days: EDD – (28 days – actual length of cycle) = EDD
As you can see there are LOTS of ways providers ‘date’ a pregnancy. Now which one is accurate, if any?
The Accuracy of ‘Due Dates’
While due dates help provide a general idea of when a baby is expected to arrive, they are not always accurate. Only 5% of babies are born on their due date!!
Most babies are born within two weeks of their due date, either before or after. Additionally, due dates can be affected by several factors– including irregular menstrual cycles, inaccurate recall of the last menstrual period, and differences in fetal growth rates.
Lots of current evidence shows that ultrasounds done in early pregnancy are more accurate than using someone’s Last Menstrual Period (LMP) to date a pregnancy. Evidenced Based Birth Article states ‘researchers found that people who had an early ultrasound to date the pregnancy were less likely to be induced for a post-term pregnancy’.
Ultrasound Dating from 11-14 weeks is said to be the most accurate way to calculate the ‘due date’. Ultrasounds performed after 20 weeks significantly decline in accuracy.
Using the Last Menstrual Period to determine a due date can be inaccurate for MANY reasons:
Women can have irregular menstrual cycles and/or cycles that are not 28 days long
Women may be uncertain about the exact date of their last menstrual period
TONS of women do not ovulate on the 14th day of their cycle
It is very common for the embryo to take longer to implant in the uterus for some women
Always remember that the due date is just an estimate, and babies can arrive earlier or later than expected. It is also important to note that not all pregnancies are the same, and some women may give birth earlier or later than others.
The Average Length of Pregnancy
There is a lottttttt of amazing research on this topic, most of which Evidenced Based Birth (referenced below) talks about in their article.
To quote the summary of their research
‘Based on the 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.’
How might you know if your pregnancy will be ‘longer’ than others? Ask your mother, grandmother, sister, partner's mother, etc about their pregnancies! Yes you heard that right–familial history is one of the biggest indicators.
Risks of Going Past Your ‘Estimated Due Date’
Often if your ‘estimated due date’ passes and you have not yet gone into labor, your healthcare provider recommends inducing labor. Most hospitals still offer inductions if a pregnancy is longer than 42 weeks.
Why induce labor? Are there risks if you go past your ‘estimated due date’?
Research suggests there are a few risks as pregnancy progresses past the 38 week mark:
Higher risk of stillbirth or neonatal death — The incidence of stillbirth or infant death is increased in pregnancies that continue beyond 42 weeks. The ACTUAL risk is relatively small, with 4 to 7 deaths per 1000 births.
Higher risk of chorioamnionitis (infection of the membranes)
Higher risk of endomyometritis (infection of the uterus)
Higher risk of having a placenta abruption (placenta separates prematurely from the uterus)
Higher risk of the baby being large at birth (>9 lbs 15 oz or >4500 grams)
Higher risk of moderate or thick meconium — Meaning the fetus had their first bowl movement when they were still in utero & run the risk of breathing this in
Each pregnancy & birth are unique. It can be hard to know whether these ‘risks’ may apply to you. It can also be hard to know if your pregnancy was ‘dated’ accurately!
We can’t always look at ‘research’ and sometimes we have to draw a conclusion for ourselves.
Work with a provider you trust & feel comfortable with. Ask alllll the questions. Do allllll the research. Everyone should have the support they deserve in pregnancy & beyond.
Conversations around due dates can be stressful! And we are each unique human beings.
We all DO NOT fit into the same category & we all definitely DO NOT have the same length of pregnancies.
If the conversation around ‘estimated due dates’ seems too stressful–don’t tell anyone! Or phrase the ‘due date’ as a ‘guess date’. You can even tell others a ‘due month’ OR ‘due season’ to keep things broad.
It is your body & your baby.
REFERENCES
Dekker, R. (2023, June 21). Evidence on: Inducing for due dates. Evidence Based Birth®. https://evidencebasedbirth.com/evidence-on-inducing-labor-for-going-past-your-due-date/
Dekker, R. (2023b, July 9). The evidence on: Due dates. Evidence Based Birth®. https://evidencebasedbirth.com/evidence-on-due-dates/
Gabe. (2022, January 11). The due date. Pathways to Family Wellness. https://pathwaystofamilywellness.org/pregnancy-birth/the-due-date.html