[Healthcare] Prorated Healthcare Coverage for Pregnancy

I have a bit of a convoluted question based on possibly flawed understandings of what I was told by a previous healthcare provider. In short, my question is: If I change insurances during pregnancy, either due to a different qualifying event like a job change or due to open enrollment at the end of the year, will the coverage provided by the new insurance be prorated?

Context: When we had our first, I was initially billed $4k+ that I did not expect. When I called the hospital and later the insurance, I was told it was a mistake and it was resolved. What had happened was that something in their system made them think that my wife had been added to my insurance during her pregnancy (not the case).

This made me wonder if the same sort of situation could happen if I had, say, UHC at the beginning of her next pregnancy, then switched to, say, Aetna for the latter part and birth. Would Aetna's coverage of the birth be prorated due to us not being on the Aetna plan for the entirety of the pregnancy?