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 (12501 total), and the shifted median if every induction (3746 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 2847 9654 13400
39W,0D 365 3212 9289 13035
39W,1D 338 3550 8951 12697
39W,2D 768 4318 8183 11929
39W,3D 458 4776 7725 11471
39W,4D 465 5241 7260 11006
39W,5D 466 5707 6794 10540
39W,6D 532 6239 6262 10008
40W,0D 1029 7268 5233 8979 Median for spontaneous births in the survey (more babies were born on or before this day than are born after)
40W,1D 607 7875 4626 8372
40W,2D 532 8407 4094 7840 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 500 8907 3594 7340
40W,4D 453 9360 3141 6887
40W,5D 487 9847 2654 6400
40W,6D 482 10329 2172 5918
41W,0D 483 10812 1689 5435
41W,1D 370 11182 1319 5065
41W,2D 297 11479 1022 4768
41W,3D 288 11767 734 4480
41W,4D 206 11973 528 4274
41W,5D 150 12123 378 4124
41W,6D 131 12254 247 3993
42W,0D 95 12349 152 3898

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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