I wanted to make this simple data table after many visitors posed this question: "We know that many women are induced for to avoid going postdate, doesn't this bias our data?"
Fair question. Most of the graphs I have just throw out inductions and show data based on only spontaneous labor. But what if ALL those women who were induced were actually supposed to go late, how long would the real average pregnancy last?
I'll just say it: Many people also ask this question because they think the medical community has an agenda and wants to induce us "early" for convenience. I do not believe this; I like doctors. They are having babies too.
288 days is actually a huge difference and it would be VERY difficult to keep using 280 days if 288 was the truth. With the 9-10 day standard deviaitons we've found, an average of 288 would mean that on day 280, 80% of pregnant women would have to still be pregnant.
Is that even possible? On this page, I pretend that every single induction before 40 weeks would have been a spontaneous birth at 42 weeks... and I still can't get to a 288 day average. Sorry, Mittendorf-Williams rule. But so many women spontaneously go into labor in the days leading up to 40 weeks, there aren't enough inductions to make a difference.
Here are the median days for the spontaneous labors (12254 total), and the shifted median if every induction (3774 total) was actually supposed to be a very late birth:
|Day||Spontaneous births on this day||Spontaneous births on or before this day||Spontaneous births after this day||Spontaneous births after this day
plus all inductions
|Before 39 W||2902||9352||13126|
|40W,0D||1040||6950||5304||9078||Median for spontaneous births in the survey (more babies were born on or before this day than are born after)|
|40W,2D||537||8101||4153||7927||Median if all inductions are added to the "later birth" total (more babies were spontaneously born on or before this day than all the ones born after, or induced)|
Conclusion: if all the induced women who took my survey had been left to stay pregnant until the very end (past 42 weeks), then the median day for birth would shift by 2 days. Not an entire week.
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
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
Survey input dates
More ResourcesPregnancy Day-by-Day
Fast pregnancy calendar
Baby age calendar
EDD Icon Generator
Jo's Birth Story
Weeks vs. Months Explained
Charts generated by flot