Statistically, When Will My Baby Be Born?

Babies aren’t very good at adhering to schedules. Especially unborn babies. Once you get to 38+ weeks pregnant, based off a very rough estimated “due date,” Due dates now seem a bit absurd to me after learning how much uncertainty there is in determining them. There’s uncertainty around ovulation timing, implantation time of the fertilised egg, accuracy of ultrasound measurements, and genetic variation between women. you are now in a limbo land of waiting. Baby could come today, or anytime within the next 4 weeks.

I’m currently in that limbo waiting phase, and spending it reading all the stats I can find on when babies tend to be born. There are surprisingly few available datasets or papers on this, but the best I could find (and get access to) was this 2001 study that looked at when 1,514 pregnant women gave birth. The data was gathered over a ten year period from 1985 and 1995 in the UK.

Taking key metrics from that, I made a tiny tool that shows a probability distribution graph of spontaneous labour starting on each day of your pregnancy. I’ve found this helped ground my expectations of when babies normally tend to arrive, and how long I can wait until I need to start worrying about induction. You can enter your own due date, and whether you’re a first-time mum, First-time mothers tend to have slightly longer pregnancies. to see your own stats:

This is based on statistical data extrapolated from the Smith 2001 paper, but don’t hold me to it too firmly. It’s only one study, I am not a data analyst, and it’s a fuzzy estimate at best. The dataset only covers 37-43 weeks, so it can’t give you predictions earlier than that.

If you’re also waiting around for a baby to arrive, hopefully you’ll find this helpful and reassuring. The TLDR is it’s very normal for it to go over 40 weeks! Risks for all the bad stuff don’t significantly increase until the end of 41 weeks. This piece on due dates and induction from Evidence Based Birth has more helpful information.