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 (12137 total), and the shifted median if every induction (3720 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 2828 9309 13029
39W,0D 364 3192 8945 12665
39W,1D 335 3527 8610 12330
39W,2D 507 4034 8103 11823
39W,3D 454 4488 7649 11369
39W,4D 463 4951 7186 10906
39W,5D 462 5413 6724 10444
39W,6D 527 5940 6197 9917
40W,0D 1013 6953 5184 8904 Median for spontaneous births in the survey (more babies were born on or before this day than are born after)
40W,1D 602 7555 4582 8302
40W,2D 526 8081 4056 7776 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 495 8576 3561 7281
40W,4D 450 9026 3111 6831
40W,5D 486 9512 2625 6345
40W,6D 476 9988 2149 5869
41W,0D 480 10468 1669 5389
41W,1D 368 10836 1301 5021
41W,2D 295 11131 1006 4726
41W,3D 283 11414 723 4443
41W,4D 202 11616 521 4241
41W,5D 148 11764 373 4093
41W,6D 130 11894 243 3963
42W,0D 94 11988 149 3869

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
Birth Stories
Survey input dates

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