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 (12355 total), and the shifted median if every induction (3812 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
Notes?
Before 39 W 2932 9423 13235
39W,0D 375 3307 9048 12860
39W,1D 346 3653 8702 12514
39W,2D 354 4007 8348 12160
39W,3D 466 4473 7882 11694
39W,4D 475 4948 7407 11219
39W,5D 477 5425 6930 10742
39W,6D 542 5967 6388 10200
40W,0D 1049 7016 5339 9151 Median for spontaneous births in the survey (more babies were born on or before this day than are born after)
40W,1D 618 7634 4721 8533
40W,2D 542 8176 4179 7991 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 512 8688 3667 7479
40W,4D 462 9150 3205 7017
40W,5D 491 9641 2714 6526
40W,6D 487 10128 2227 6039
41W,0D 494 10622 1733 5545
41W,1D 378 11000 1355 5167
41W,2D 306 11306 1049 4861
41W,3D 295 11601 754 4566
41W,4D 210 11811 544 4356
41W,5D 156 11967 388 4200
41W,6D 136 12103 252 4064
42W,0D 97 12200 155 3967

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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