> in the United States during 2018-2020 (average), the highest fertility rates per 1,000 women were to women ages 20-29 (80.1), followed by women ages 30-39 (75.3) [...]
Part of the issue is the infertility cliff happens in the late 30s to mid 40s, so those age categories are a little wonky in terms of capturing the phenomenon.
Having bumped up against this myself, the problem in the US isn't that women are more often waiting until they are 32 to have kids, it's that they are waiting until they are 39.
There seems to be a much bigger decline in births among younger than there is an increase in older, but if those women are delaying rather than abstaining completely, they'll run into infertility issues later if they wait late enough. There will be a certain number who intend to have more kids than they actually do.
I don't think this is all of it, as it doesn't address why women are increasingly forgoing children in prime fertility years. I also in general think it's part of a set of demographic trends that point to societal problems (stalling or declining life expectancy for one).
Note that these age-specific fertility rates measure the number of births per 1,000 women. This does not directly measure how easy it is for a woman in that age group to conceive (what OP referenced), because fertility rate is confounded by the degree to which women (1) actively try to conceive (versus avoid conception), and (2) decide to carry pregnancies to term.
Source: https://www.google.com/search?client=safari&rls=en&q=birth+r...