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?"

Fair question. 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?

Is the Mittendorf-Williams rule be more accurate?

Most people who ask me if 288 days is the real average have read about the "Mittendorf Rule", also known as the "Mittendorf-Williams" rule. This is based on one 1990 Harvard study called The length of uncomplicated human gestation (Mittendorf et al). The study looked at 31 first-time moms at one private practice, and their average due date was eight days early. If you want to read it, it's here. But I have not found any other study that duplicated thier results, and I've found multiple other studies (noted on the intro page) that recommend sticking with 40 weeks (280 days), changing the average by maybe a day or two for first time moms. I separate out my survey data for first time moms here.

I'll just say it: Many people also ask this question because they think the medical community has an agenda and wants to induce us "early" for convenience. I do not believe this; I like doctors. They are having babies too.

288 days is actually a huge difference and it would be VERY difficult to keep using 280 days if 288 was the truth. With the 9-10 day standard deviaitons we've found, an average of 288 would mean that on day 280, 80% of pregnant women would have to still be pregnant.

Is that even possible? On this page, I pretend that every single induction before 40 weeks would have been a spontaneous birth at 42 weeks... and I still can't get to a 288 day average. Sorry, Mittendorf-Williams rule. But so many women spontaneously go into labor in the days leading up to 40 weeks, there aren't enough inductions to make a difference.

Here are the median days for the spontaneous labors (12254 total), and the shifted median if every induction (3774 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 2902 9352 13126
39W,0D 372 3274 8980 12754
39W,1D 341 3615 8639 12413
39W,2D 350 3965 8289 12063
39W,3D 464 4429 7825 11599
39W,4D 470 4899 7355 11129
39W,5D 472 5371 6883 10657
39W,6D 539 5910 6344 10118
40W,0D 1040 6950 5304 9078 Median for spontaneous births in the survey (more babies were born on or before this day than are born after)
40W,1D 614 7564 4690 8464
40W,2D 537 8101 4153 7927 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 508 8609 3645 7419
40W,4D 460 9069 3185 6959
40W,5D 490 9559 2695 6469
40W,6D 485 10044 2210 5984
41W,0D 491 10535 1719 5493
41W,1D 374 10909 1345 5119
41W,2D 304 11213 1041 4815
41W,3D 294 11507 747 4521
41W,4D 208 11715 539 4313
41W,5D 155 11870 384 4158
41W,6D 135 12005 249 4023
42W,0D 96 12101 153 3927

Conclusion: 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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