September 25th, 2010


Pregnancy, Due Dates, and the length of Human Gestation: My Statistics

Several of you may remember that towards the end of my pregnancy, I got really interested in statistics of due date accuracy and human gestation. I researched, made tables, got mad at data... and finally launched my own online survey to ask women who'd had babies when they had them. And then JUST when the data was getting interesting I went and had a baby and couldn't really look at the results. But the survey didn't die, and I've been meaning to post a sort of epilogue about what I found.

The short story: that "40 weeks" idea is a pretty damn good guess at the average. I mean seriously. When I looked at just the 141 spontaneous deliveries after 37 weeks ("term"), both the mean and median date fell right smack on 280 days, or 40 weeks 0 days. This means a woman is equally likely to have her baby before the date than after it. And the most common days to have a baby are all clustered around that date, which lends to the theory that we can apply a normal curve to this (I love normal curves).

But I'd like to explore all this a lot more, not just my statistics but my reasons for the survey and answers to frequent questions/criticisms. Let's go for a ride, shall we?

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