Due Dates Don't 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.