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Many doctors and midwives have strict restrictions on how far past the due date you can go before they'll induce labor. Some let you go 1 week, others 2 weeks. Many natural birth advocates claim that this is somewhat bogus because due date predictions are very rough estimates and are subject to a lot of change. I'm not educated on the methodology of predicting due dates to have an educated opinion, so I thought I'd put it to the community...

Are due dates an accurate measurement of babys maturity (or over-maturity)? Has their accuracy improved over time with new methods? Should they relied on any more than a rough guestimate of when the baby has become mature or over mature to the point of requiring induction?

(I'm not trying to incite a flamewar over natural child birth, I just want facts)

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I don't have scientific data, so I'll just add it as a comment: AFAIK, 50% of the children are born in the time interval +/- 2 weeks around the calculated date of birth. (That's what the gynaecologist and the midwives told us) –  BBM Nov 30 '11 at 22:50
    
... and only 4% are born on the exact due date. (source: my pediatrician) –  Torben Gundtofte-Bruun Dec 1 '11 at 8:37
    
Are you asking if the baby's "age" (number of weeks since conception) can be accurately determined, or if all babies mature at the same rate and will be perfectly ready at 40 weeks? –  Rachel Apr 5 '12 at 4:00
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If they are using the last menstrual date as the only indication than it is potentially very inaccurate as many women have irregular cycles. However if the mother is receiving regular ultrasounds your doctor should be able to correct with good accuracy for this. They may not be able to give an exact conception date, but close enough to make an accurate decision. Beyond that though they are going to largely rely on their personal and institutional preference when deciding exactly how long to wait.

As far as if all babies are "done" in 40 weeks of gestation, that's quite a bit more complicated most fetuses will have a high survival rate after they reach around 1 KG and around 26 weeks gestational age, though some neonatal intervention would generally be required. After about 37 weeks it's really about not interfering with the natural process unless there seems to be a pressing need since there's a great deal of research leading to the conclusion that outcomes improve when intervention is minimized.

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To clarify a question on the answer regarding the accuracy of ultrasounds. Ultrasounds will generally measure bone length, not soft tissue, so estimates of weight assume an average distribution of tissue which can be a somewhat crude method. Age as measured by factors such as fetal pole length or generally more accurate. –  yelkereb Dec 1 '11 at 1:11
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Although initially they go by a fixed number of weeks based on the last menstrual cycle, my wife's doctor adjusted the due date once or twice based on the actual size of the fetus. That tells me they estimate the due date based on the baby's maturity rather than the other way around.

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On the other hand, the size of the fetus that they determine based on the ultrasounds is a ballpark guess at best: a friend of mine recently had a baby that the doctors estimated as close to 10 pounds, but he was actually less than 7 pounds. –  Martha Dec 1 '11 at 0:44
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