Comment The big drop was the 1960s (Score 1) 176
https://www.macrotrends.net/global-metrics/countries/usa/united-states/fertility-rate
https://www.macrotrends.net/global-metrics/countries/fin/finland/fertility-rate
That drop alone brought the US down to about the current total fertility rate: 1960 (3.65), 1973 (1.88), 2024 (1.79). The drop from 2007 (2.12) until now is pretty minor in comparison. I of course won't say that birth control is the only thing that changed in the '60s: a lot of other factors starting converging then as well.
Overall I think the biggest contributor to the drop has been in unintended pregnancies: rates dropped the furthest in teenagers up to age 24. Most people, rich or poor, when they have a real choice, choose to 1, wait to have kids, 2, have fewer of them.