I’ve heard of so many people going past 40 weeks with at least one of their pregnancies. Is this really that common? And what causes a baby to be born after the due date?
—Ready to Pop
We do not perfectly understand why labor starts at different times for different people — or even what specifically triggers it. Some combination of hormones from the fetus and the placenta, some signal of fetal lung maturity, and possibly pressure on the cervix are all part of the process. But predicting the timing of labor is very challenging. By extension, predicting who will have a preterm birth and who will go longer than their due date is also a challenge.
There are, however, a few consistent things that correlate with longer pregnancy.
The first is a mistake about when the pregnancy started. Pregnancy dating is notoriously complicated, and if you aren’t sure when you conceived, the due date might be wrong by a few days. This could produce an error in either direction, but it’s one reason that you might deliver at what appears to be “post-term.”
Second, first births tend to take longer. If this is your first baby, you’re more likely to give birth later in general, including after the due date.
There are a few other correlates: older mothers seem to give birth a bit later on average, and white women are more likely to give birth post-term relative to women of other races. Having said this, none of these variables are very predictive — a lot of it seems to be about individual variation. One strong predictor is having gone post-term with an earlier baby, suggesting that there is something about some women that makes this more likely. And if you yourself were born post-term, you are more likely to have a post-term birth.
One note: Doctors are increasingly less likely to let pregnancies continue post-term. Most providers will strongly encourage induction at or after 41 weeks, if not before. By this point, it may be harder to argue against an induction for one.
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What about a second pregnancy after not having actually gone in to labor the first time around (specifically, planned c-section for first birth due to breech baby)?