…Though to be clear, it’s nearly impossible for it to be right (aka predict the exact day your baby will be born).
How did we get here?
The conventional 40 week due date your OB will cling to has very little grounding in reality. It was suggested in the mid-1700s by a man who observed 100 pregnancies. From there, the concept was adjusted/expanded upon by another man, Naegele, in the 1800s, leading to the due date calculation we know today based upon Naegele’s Rule. The general idea is to add seven days to your last period and then add nine months.
Voilà! The day your baby will come! Or is it….?
Beyond lacking an even remotely decent sample size, both men weren’t very clear about how they did their calculations. Neither said if their calculations – last period plus seven days plus nine months – began with the first or the last day of a woman’s period. They also assumed a 28 day cycle which is not always the case for women, and they also didn’t really account for differing length months in the year. Let’s also not forget the wild ride the sperm and egg take to meet and implant and the variations in timing of that whole journey!
It was thought for a very long time that it was intended to be from the last day of the last menstrual period, but for some reason in modernity (starting around 1900), the calculation was pushed to using the first day. This means a baby’s due date can have a two or three week swing depending on the calculation starting on the first or last date of the period.
Today’s due date calculations are based on basically no evidence whatsoever. What current evidence is showing is that an early dating ultrasound tends to be more accurate than a calculation based on the last period, but even that has its flaws (not to mention the debate over ultrasound safety).
Ok, but who cares?
So why does it matter? When most women are birthing in a system that puts a LOT of stock into due dates, the validity of the idea matters tremendously. Pressure builds from 37 weeks onward for inducing labor for fear of baby becoming “too big,” baby being “too small” and needing help, the placenta aging, and other perceived issues of a fully grown baby being birthed exactly how nature intended. The ARRIVE trial is also used as a way to encourage early induction because it showed (in an extremely limited population with a pretty flawed study otherwise) that 39 week inductions reduced the number of c-sections.
But if the way we calculate a due date is wrong, how can we so confidently convince women to pull their growing baby from her belly before baby is ready? Statistics, as skewed as they may be, show that due dates are no more than guess dates, the average baby coming earthside between 40+3 and 40+5 depending if you’re a first time mama or not. (Studying this is hard because of the massive amounts of induction – baby’s gestation is prematurely cut off, so we don’t know what naturally would have occurred without intervention.) The VAST majority of women give birth by 41+3 and nearly every woman will deliver by 42 weeks.
This conversation also brings up the interesting question – is pregnancy even 40 weeks long? Yes and no… There have been a few studies that show pregnancies last closer to 41 weeks, not 40. This is especially true for first time mamas.
Rather than a due date, a due month or due period would likely serve women better. Reducing the anxiety, bullying, and fear surrounding something nature so beautifully chooses – your baby’s birth date – can be done easily by ignoring two men from the 16th and 17th centuries and instead trusting and listening to the women carrying the child
TTC? Just found out you’re pregnant? In the middle of a pregnancy and want to get in the right mindset for birth? Go check out my pregnancy freebie to help you think through your options!