Probability of delivery within x days of a given date

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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 674 out of 15223 (4.4%) spontaneously arrived on the exact day. But if you look around that date, the numbers get bigger fast... 3729 babies (24.5%) arrived spontaneously between 39 weeks 4 days and 40 weeks 3 days. 7157 (47%) arrived spontaneously within a week of their due date. Throwing out inductions makes those percentages even higher: 61.7% 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 61.7% 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 60.3% chance of delivering within a week of that "wrong" date. It's not really a huge difference.

Data Table

DayIn weeksBabies Spontaneously Born
24535W,0D21
24635W,1D21
24735W,2D22
24835W,3D25
24935W,4D24
25035W,5D27
25135W,6D25
25236W,0D44
25336W,1D30
25436W,2D44
25536W,3D47
25636W,4D37
25736W,5D48
25836W,6D60
25937W,0D87
26037W,1D84
26137W,2D83
26237W,3D97
26337W,4D116
26437W,5D115
26537W,6D152
26638W,0D187
26738W,1D159
26838W,2D172
26938W,3D221
27038W,4D282
27138W,5D255
27238W,6D297
27339W,0D362
27439W,1D334
27539W,2D402
27639W,3D448
27739W,4D460
27839W,5D461
27939W,6D524
28040W,0D674
28140W,1D599
28240W,2D523
28340W,3D488
28440W,4D446
28540W,5D485
28640W,6D476
28741W,0D475
28841W,1D365
28941W,2D291
29041W,3D281
29141W,4D202
29241W,5D148
29341W,6D130
29442W,0D94
29542W,1D42
29642W,2D31
29742W,3D28
29842W,4D13
29942W,5D13
30042W,6D10
30143W,0D12

Due Date Survey Data

Due 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
Are more babies born during a full moon?
What's the most common day of the week for babies to be born?
I'm still pregnant at 40W. What's it mean?
Do winter babies arrive later?
What if I know my conception date?
Are boys or girls born later?
Subsequent pregnancy date search
How do inductions bias the due date statistics?
Download the data
Birth Stories
Survey input dates

More Resources

Pregnancy Day-by-Day
Fast pregnancy calendar
Baby age calendar
EDD Icon Generator
Jo's Birth Story
Weeks vs. Months Explained
Charts generated by flot
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