Spontaneous labor and due date determination

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This survey assumed that all due dates were set at 280 days, or 40 weeks, past last menstrual period (LMP). If LMP wasn't used, 280 was still the due date marker. For example if the baby came one day after its due date, I assumed that was day 281.

The graph & table on this page show how close the different methods of determination were. The closer they were to a 280 day average, the more accurate. So far no method seems to be dramatically more accurate than just counting 40 weeks past LMP.

Method of Due
Date Determination
# of births Average Arrival Average Error (days) Standard Deviation
LMP533139W,5D 2 9.4
ovulation117439W,6D 1 9.2
ultrasound499439W,5D 2 9.8
Other9939W,5D 2 10.4

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