Economic ability to pay for these medical treatments (well, in the U.S. at least), as well as all the other things needed to keep the lions away from the kids. There's a big fertility bust among what used to be the middle class, as many people don't see how they could possibly afford the necessities needed to raise a child (health care, housing, education, safe environment, parental attention) as they were raised.

(I suspect that there's also simply less selection pressure, in the sense that more people are having 1 or 2 kids, vs. in historical times where the emperor my have had 1000, a wealthy man dozens, and lots of paupers ended up with no kids at all. That has the effect of letting lots of genetic variation into the genome without immediately weeding it out, at least until some selection event occurs in the future.)