I’m just not sure if its a statistic that’s been kept. Pregnancy length has, until recently, been a best educated guess, since older tests weren’t acurate before 12 weeks.
The early term tests have only been around for a few years. They will become positive 14 days after implantation. But, there’s still no way to tell how much more than 14 days have gone since implantation.
As I said eariler, Those 48 weekers were very possibly unrecognized early miscarriages with later re-fertilization.
Say, the woman does an early term test. Its positive. A month, or more goes by, she has some spotting, but it stops. The signs wouldn’t be dramatic, and those that are apparent, don’t disappear over night. Because she assumes she is pregnant, she continues to have unprotected sex, and is re-fertilized.
The doctor takes the word of the mom as to when her last period was, then calculates the due date. No real science.
If you look at the animal model, all placental mammals have very circumscribed gestational periods, with afew exceptions, of course. Some species will re- absorb a gamete if conditions are not acceptable.
Marsupials, on the other hand, function a little differently, in that the development can slow, until conditions are optimal. They are, I believe, the only mammals with variable lengths of gestation.
I’ve cared for a few post term infants. The most post term was 43 weeks. The baby’s skin was cracked, with some areas of necrosis. He had fairly severe brain injury.
The OB told us that the placenta was only about 1/4 viable, the rest was black.
Sorry I can’t help.