Many online resources state that it's very unlikely for a pregnancy to result in a delivery exactly on the estimated due date. I've seen the number of about 5%, but can't find it nor am much interested in this information specifically.

There is also a popular opinion saying that the first pregnancy natural labor usually happens after a due date rather than on or before it.

As you can tell, I am a bit skeptical about it. Could anyone point me to an official source that has the information on the distribution of terms of the natural labors in the first pregnancies?

Here's a link to a site that says:

Babies don't always arrive when they're supposed to, and mothers often say firstborns tend to be late, although this conclusion is typically based on anecdotal reports

He [Allen Downey] found that first babies were, indeed, slightly more likely to be born late, at 41 weeks or later, compared with other babies (second-borns, third-borns, etc.)

I do not know if this question is location related. But for the sake of specificity you can assume I'm asking about the United States.

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    Just noting that a quick search shows this to be notable. – Oddthinking Oct 24 '17 at 3:35
  • @Oddthinking The opinion is expressed by pretty much every single mother under age 30 my wife and I have talked to. Does Skeptics.SE require a source to be "documented" in order to make the question valid? I apologize in advance if that is the case and I missed it. – Igor Soloydenko Oct 24 '17 at 3:38
  • @Oddthinking Also, there's plenty of stats on pregnancy duration in general case, but my question is specifically about the first pregnancies. – Igor Soloydenko Oct 24 '17 at 3:39
  • Sorry, I removed the comment shortly after I posted it. In general, yes, we require the source to be documented to show it is notable. When I searched to find something to edit into the question, what I found was lots of people with scientific evidence about the answer, but no-one actually making the claim without evidence. – Oddthinking Oct 24 '17 at 3:42
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    @oddthinking yeah, and that's exactly what I'm asking for -- more precise source that shows raw data, or study explained in details. – Igor Soloydenko Oct 24 '17 at 12:18

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