Anti-Natal Care: The Due Date Fallacy

More on Estimated Due Dates (EDD)

The “standard algorithm” to calculate gestational due date is called Naegele’s Rule. It was formulated in 1850.

Fast forward to 1990 for the first ever update to the due date calculation, following some studies showing that Caucasian women deliver most often a good 8 days after just about everybody else, and babies of first-time mothers almost always take an extra week over a multiparous woman’s gestation length. Now there’s a Mittendorf calculation, named after the guy who realized there’s a racial and experiential difference in gestation lengths.

(What that means for me: if you’re white and it’s your first kid, you take your EDD from Naegele’s Rule and add 8 days. Brilliant.)

There’s more to it than that, actually; there’s a software model that can be used to take 16 of factors into account like smoking, maternal age, and even college education, to reach a gestational prediction that’s twice as accurate as Naegele’s Rule.

But even with all the tech-mology we’re throwing at this, women still only have roughly a 29-37% chance of delivering in the 40th week! And the Mittendorf study actually shows the average length of human gestation is 41 weeks + 1 day.

I understand there are statistically more risks in carrying babies longer than 42 weeks. The placenta has an expiration date, and the baby’s going to have his first poop at some point and you don’t want him to do it inside the womb. These are aspects of what’s known as “post-maturity syndrome”.

But if the predictions we’re using aren’t actually improving fetal outcomes, then we’re most likely causing undue anxiety and unnecessarily early inductions in a significant number of pregnancies where there should have been no concern about post-maturity syndrome at all. Indeed, researcher Nichols study that “70% of women classified as postdate in his study were incorrectly dated.”

Add on the fact that only a small percentage of pregnancies carried beyond 42 weeks result in babies born with post-maturity syndrome — 95% of babies born at gestational ages between 42 and 44 weeks are born safely and without incident — and a picture emerges of a cultural (ours) in which habitual early induction is committed ritualistically, unscientifically, and with only the shakiest of indications to support it.

I wouldn’t be so bothered about all of this but for two big issues, one personal and one societal. Obviously I have a vested interest in doing a natural, non-medically-induced birth, and that means — much to my ire — getting “permission” from the medical establishment to carry this child until he’s good and ready to come out, on pain of not being admitted to the hospital of my choice if I choose to continue my pregnancy “post term”.

(St. Luke’s will not accept any pregnancies past 41 weeks + 6 days; if you are in their care, they will schedule you an induction on or before that day — whatever They calculate that day to be.) Yes, I’m doing a home birth, but my midwife is beholden to the medical world in many ways, one of them being the policies of your back-up hospital in case something goes wrong at home.)

But when I read the attitudes and stories of women who post in the forums on babycenter.com (my sort-of-guilty pleasure for the past 10 months), in which the dominant attitude seems to be very much pro scheduled inductions and C-sections, that’s when I get really mad. Because I know that these women have been guilted and lied to, misled about the supposed equivalence of these procedures to a natural birth, made anxious about the long-term effects of vaginal delivery (incontinence is not irreversible OR inevitable!), and encouraged to think of their gestation length as a fool-proof, scientifically calculated finish line…

…when really, the medical establishment has been routinely using a demonstrably faulty technique from 1850 to decide pregnancy outcomes.


This article, referencing several medical studies on human gestation length and perinatal outcomes, was enormously helpful in writing this blog post:

Calculating Due Dates and the Impact of Mistaken Estimates of Gestational Age

And this Canadian study on post-term pregnancies provided a lot of useful statistics:

Recent Trends in Fetal and Infant Outcomes Following Post-term Pregnancies

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