I saw biological science film that showed that, when survival rates are high, the birth rates tend to drop. It’s a perfectly natural response to being assured that the progeny will make it to adult hood.
Well, as you say, it was a SCIENCE film, so no thinking person could find fault with its unexamined assumptions and unsupported conclusions.
Once birth rates drop below about 2.1 children per woman, though, you’ve crossed the line into slow societal suicide.