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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 265 out of 6988 (3.8%) spontaneously arrived on the exact day. But if you look around that date, the numbers get bigger fast... 1688 babies (24.2%) arrived spontaneously between 39 weeks 4 days and 40 weeks 3 days. 3214 (46%) arrived spontaneously within a week of their due date. Throwing out inductions makes those percentages even higher: 60.8% 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 60.8% 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 59.5% chance of delivering within a week of that "wrong" date. It's not really a huge difference.

Data Table

DayIn weeksBabies Spontaneously Born
24535W,0D9
24635W,1D11
24735W,2D8
24835W,3D16
24935W,4D8
25035W,5D13
25135W,6D12
25236W,0D22
25336W,1D17
25436W,2D24
25536W,3D26
25636W,4D21
25736W,5D29
25836W,6D25
25937W,0D35
26037W,1D33
26137W,2D36
26237W,3D59
26337W,4D60
26437W,5D51
26537W,6D77
26638W,0D79
26738W,1D72
26838W,2D71
26938W,3D110
27038W,4D130
27138W,5D128
27238W,6D138
27339W,0D166
27439W,1D148
27539W,2D154
27639W,3D216
27739W,4D213
27839W,5D221
27939W,6D260
28040W,0D265
28140W,1D262
28240W,2D244
28340W,3D223
28440W,4D208
28540W,5D215
28640W,6D204
28741W,0D215
28841W,1D150
28941W,2D121
29041W,3D125
29141W,4D96
29241W,5D77
29341W,6D60
29442W,0D48
29542W,1D20
29642W,2D18
29742W,3D10
29842W,4D9
29942W,5D6
30042W,6D1
30143W,0D7

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?
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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