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 (3238 total), and the shifted median if every induction (1031 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		757	2481	3512
39W,0D	107	864	2374	3405
39W,1D	80	944	2294	3325
39W,2D	94	1038	2200	3231
39W,3D	127	1165	2073	3104
39W,4D	123	1288	1950	2981
39W,5D	136	1424	1814	2845
39W,6D	156	1580	1658	2689
40W,0D	283	1863	1375	2406	Median for spontaneous births in the survey (more babies were born on or before this day than are born after)
40W,1D	167	2030	1208	2239
40W,2D	146	2176	1062	2093	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	131	2307	931	1962
40W,4D	120	2427	811	1842
40W,5D	116	2543	695	1726
40W,6D	114	2657	581	1612
41W,0D	135	2792	446	1477
41W,1D	98	2890	348	1379
41W,2D	65	2955	283	1314
41W,3D	69	3024	214	1245
41W,4D	59	3083	155	1186
41W,5D	41	3124	114	1145
41W,6D	32	3156	82	1113
42W,0D	35	3191	47	1078

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? 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 spacefem.com spacefem@spacefem.com

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