How do inductions bias the due date statistics?

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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? After all, 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? Most of the people asking this question believe in their hearts that the average baby wants to arrive closer to 41 weeks or later, and the medical field is in denial and wants to induce us for convenience. I do not believe this, but wanted a quick way to explain myself, so here's this page.

The problem with the "we're all meant to go 41 weeks" theory is that so many women spontaneously go into labor in the days leading up to 40 W, there aren't enough inductions (even in our induction-happy world) being done to offset the average. Here are the median days for the spontaneous labors (12314 total), and the shifted median if every induction (3735 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 2840 9474 13209
39W,0D 365 3205 9109 12844
39W,1D 337 3542 8772 12507
39W,2D 611 4153 8161 11896
39W,3D 458 4611 7703 11438
39W,4D 464 5075 7239 10974
39W,5D 465 5540 6774 10509
39W,6D 531 6071 6243 9978
40W,0D 1021 7092 5222 8957 Median for spontaneous births in the survey (more babies were born on or before this day than are born after)
40W,1D 607 7699 4615 8350
40W,2D 531 8230 4084 7819 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)
40W,3D 499 8729 3585 7320
40W,4D 453 9182 3132 6867
40W,5D 486 9668 2646 6381
40W,6D 480 10148 2166 5901
41W,0D 483 10631 1683 5418
41W,1D 368 10999 1315 5050
41W,2D 296 11295 1019 4754
41W,3D 288 11583 731 4466
41W,4D 206 11789 525 4260
41W,5D 149 11938 376 4111
41W,6D 131 12069 245 3980
42W,0D 95 12164 150 3885

So basically, 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 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
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Survey input dates

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