Animals have "r/k selection": https://en.wikipedia.org/wiki/R/K_selection_theory ; some have huge numbers of offspring (e.g. spiders, most fish), some carefully nurture a single egg per year. Humans are already at the smaller number of offspring compared to the rest of the animal kingdom, but what I think is happening is that social pressure has simply pushed the tradeoff hard into "quality".

That is, the message is "unless you can give your children a perfect life, you shouldn't bother".

Certainly the main victory against birthrate worldwide has been the long process of eradicating teen pregnancy.

> Easy access to contraceptives probably makes a significant difference too

This is so basic as to be an axiom of the whole thing. The politics of going back to forced childrearing through suppression of healthcare are horrific, but some of the US is pushing for that.