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
Abstract
Current estimates of the average duration of human pregnancy are flawed by inaccurate estimation of the time of conception and by failure to account adequately for the effect of routine elective delivery post-term. METHODS: In this study, 1514 healthy pregnant women were studied in whom the discrepancy between the menstrual history and first trimester crown–rump length estimated gestational age was within –1 to +1 day difference. The duration of gestation was estimated using time to event analysis: non-elective delivery was taken to be the event, and elective delivery was taken to be censoring. RESULTS: The median time to non-elective delivery using the Kaplan–Meier product limit estimate was 283 days after last menstrual period (LMP) and there was no difference comparing male and female fetuses. The median was significantly greater for nulliparous women compared with multiparous women (284 versus 282 days, P < 0.0001). Multivariate analysis using Cox's proportional hazards model confirmed the independent effect of nulliparity on duration of pregnancy [hazard ratio, 0.75; 95% confidence interval (CI) 0.67–0.85] and demonstrated no effect of maternal age, previous abortions, fetal sex, high parity, or bleeding before 24 completed weeks of gestation. Bleeding in the third trimester of pregnancy was, however, associated with an earlier onset of spontaneous labour (hazard ratio, 1.38; 95% CI 1.03–1.84). CONCLUSION: This study provides a basis for predicting the probability of labour at a given gestational age at term
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. The dataset only covers 37-43 weeks of pregnancy, so it can’t give you predictions earlier than that. 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:
You are -- weeks and -- days pregnant, and --. You have a -- chance of giving birth by tomorrow.
Cumulative Birth Probability
The daily birth probability is the chance of giving birth on that specific day. The cumulative birth probability is the chance of having given birth by that day.
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.
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.