How do inductions bias the due date statistics?

[ Survey Results Intro | All Charts | Take the survey ]

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 (11999 total), and the shifted median if every induction (3697 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 2813 9186 12883
39W,0D 364 3177 8822 12519
39W,1D 334 3511 8488 12185
39W,2D 424 3935 8064 11761
39W,3D 450 4385 7614 11311
39W,4D 460 4845 7154 10851
39W,5D 461 5306 6693 10390
39W,6D 526 5832 6167 9864
40W,0D 1006 6838 5161 8858 Median for spontaneous births in the survey (more babies were born on or before this day than are born after)
40W,1D 600 7438 4561 8258
40W,2D 524 7962 4037 7734 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 489 8451 3548 7245
40W,4D 449 8900 3099 6796
40W,5D 485 9385 2614 6311
40W,6D 476 9861 2138 5835
41W,0D 476 10337 1662 5359
41W,1D 366 10703 1296 4993
41W,2D 292 10995 1004 4701
41W,3D 281 11276 723 4420
41W,4D 202 11478 521 4218
41W,5D 148 11626 373 4070
41W,6D 130 11756 243 3940
42W,0D 94 11850 149 3846

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

More Resources

Pregnancy Day-by-Day
Fast pregnancy calendar
Baby age calendar
EDD Icon Generator
Jo's Birth Story
Weeks vs. Months Explained
Charts generated by flot