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