Due Dates Don't Actually Predict 'Normal' Length of Pregnancy

Illustration for article titled Due Dates Dont Actually Predict Normal Length of Pregnancy

Setting the oven timer to 40 weeks doesn't work for every bun. According to a new study, due dates aren't always practical or helpful as healthy pregnancies can vary naturally up to five weeks.

Due dates are calculated using the first day of a woman's last period and then adding 40 weeks. It's general knowledge that this is only an ETA and not set in stone. Only 4% of women deliver on their due date, while 80% deliver either two weeks before or two weeks after. But certain protocol has been established using the magical 40 week mark. For instance almost all doctors will induce at 42 weeks, since they consider that a "prolonged pregnancy."

However, a study published in the journal Human Reproduction has found that healthy pregnancies range in length from 38 - 43 weeks. A team at the US National Institute of Environmental Health Sciences studied daily urine samples of 125 women trying to become pregnant naturally. Using hormone levels in the urine, they were able to pinpoint the exact date of conception. Excluding premature births, they found that the gestation period varied of the women by as much as 37 days. According to one of the researchers:

We were a bit surprised by this finding. We know that length of gestation varies among women, but some part of that variation has always been attributed to errors in the assignment of gestational age.

Our measure of length of gestation does not include these sources of error, and yet there is still five weeks of variability. It's fascinating.


This could mean that some women are being unnecessarily induced because their internal clocks don't align with guesswork that is considered current standard.

Image via Alessandro Colle/Shutterstock

The 40-week pregnancy myth has popped [The Guardian]

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Ari Schwartz: Dark Lord of the Snark

I actually asked Dr. Wife why doctors bother inducing after due dates (doesn't the body just naturally do shit... naturally?) and she said that the baby can get too big to birth and it actually starts ingesting meconium (baby shit.)

So I just googled it and:

"Why the concern after two weeks? The longer your pregnancy continues, the larger your baby is likely to be — which might complicate a vaginal delivery. In a few cases, aging of the placenta might compromise a baby's ability to thrive in the womb. An overdue baby is also more likely to inhale fecal waste (meconium) during childbirth, which can cause breathing problems or a lung infection after birth."

So while some women may be unnecessarily induced, there are definite good reasons here. I wonder if maybe there are better ways to personalize this though? "Personal medicine" is the new buzzword these days, so might as well go for it!