This graph shows only babies born as a result of spontaneous (non-induced) labor (76% of all participants).
Do babies come on their due dates? Not really. But they seem to come around their due dates. In the survey, only 240 out of 5835 (4.1%) spontaneously arrived on the exact day. But if you look around that date, the numbers get bigger fast... 1435 babies (24.6%) arrived spontaneously between 39 weeks 4 days and 40 weeks 3 days. 2741 (47%) arrived spontaneously within a week of their due date. Throwing out inductions makes those percentages even higher: 61.6% of all non-induced babies arrived within a week of their due date.
I also think this graph is important because it shows how meaningless it is to move a due date one or two days. For example I have a chart comparing first time moms to mothers who've had previous births, and it shows about a 1-2 day difference. But based on this graph, who cares about a day?
What I mean is this: a woman who's not induced has a 61.6% chance of giving birth within a week of her due date. Let's say we lie to her and tell her a date that's three days early. Now she has a 60.4% chance of delivering within a week of that "wrong" date. It's not really a huge difference.
|Day||In weeks||Babies Spontaneously Born|
Due Date Survey DataDue date statistics: A study on the length of pregnancy
Probability of delivery resulting from spontaneous labor after 35 weeks
Probability of delivery within x days of a given date
Length of pregnancy by week
Spontaneous labor and due date determination
Length of pregnancy, comparing subsequent births for individual moms
Length of pregnancy for first time vs. second & third time moms
Length of pregnancy, type of delivery
Gestation vs. Birthweight
Probablity of Induction after a given day
Average day of spontaneous labor vs. age of mother at time of birth
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Subsequent pregnancy date search
How do inductions bias the due date statistics?
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