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 (11852 total), and the shifted median if every induction (3679 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 2796 9056 12735
39W,0D 364 3160 8692 12371
39W,1D 330 3490 8362 12041
39W,2D 344 3834 8018 11697
39W,3D 446 4280 7572 11251
39W,4D 458 4738 7114 10793
39W,5D 459 5197 6655 10334
39W,6D 521 5718 6134 9813
40W,0D 1000 6718 5134 8813 Median for spontaneous births in the survey (more babies were born on or before this day than are born after)
40W,1D 597 7315 4537 8216
40W,2D 521 7836 4016 7695 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 486 8322 3530 7209
40W,4D 446 8768 3084 6763
40W,5D 483 9251 2601 6280
40W,6D 475 9726 2126 5805
41W,0D 472 10198 1654 5333
41W,1D 364 10562 1290 4969
41W,2D 290 10852 1000 4679
41W,3D 280 11132 720 4399
41W,4D 202 11334 518 4197
41W,5D 146 11480 372 4051
41W,6D 130 11610 242 3921
42W,0D 94 11704 148 3827

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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