Contraceptives and easy access to them are definitely a big part of it. The largest factor in birth rates is access to education for women. Higher levels of schooling lead to delayed marriage, later childbearing, and fewer total children.

https://pmc.ncbi.nlm.nih.gov/articles/PMC3449224/

https://wol.iza.org/articles/female-education-and-its-impact-on-fertility/long

[–] CanadaPlus@lemmy.sdf.org 2 points 1 day ago* (last edited 1 day ago)

Yep, and that's probably through better access to and knowledge of contraception and other kinds of family planning. In really patriarchal societies men don't have to worry as much about the difficulty of raising a high number children. Or giving birth to them.

You can find anecdotes supporting that causation, although there's no hard data that I'm aware of.