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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 250 out of 6146 (4.1%) spontaneously arrived on the exact day. But if you look around that date, the numbers get bigger fast... 1508 babies (24.5%) arrived spontaneously between 39 weeks 4 days and 40 weeks 3 days. 2864 (46.6%) arrived spontaneously within a week of their due date. Throwing out inductions makes those percentages even higher: 61.2% 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 61.2% 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 60.1% chance of delivering within a week of that "wrong" date. It's not really a huge difference.

Data Table

DayIn weeksBabies Spontaneously Born
24535W,0D6
24635W,1D6
24735W,2D6
24835W,3D13
24935W,4D5
25035W,5D12
25135W,6D10
25236W,0D19
25336W,1D16
25436W,2D23
25536W,3D22
25636W,4D15
25736W,5D24
25836W,6D24
25937W,0D31
26037W,1D25
26137W,2D37
26237W,3D54
26337W,4D50
26437W,5D47
26537W,6D65
26638W,0D66
26738W,1D63
26838W,2D60
26938W,3D97
27038W,4D107
27138W,5D115
27238W,6D121
27339W,0D147
27439W,1D130
27539W,2D139
27639W,3D187
27739W,4D186
27839W,5D197
27939W,6D231
28040W,0D250
28140W,1D240
28240W,2D216
28340W,3D188
28440W,4D192
28540W,5D187
28640W,6D182
28741W,0D192
28841W,1D142
28941W,2D109
29041W,3D113
29141W,4D82
29241W,5D70
29341W,6D52
29442W,0D46
29542W,1D19
29642W,2D16
29742W,3D7
29842W,4D7
29942W,5D6
30042W,6D0
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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