When we see major drops in birth rates over a short period of time it will triggered by something – most recently the Global Financial Crisis, and currently the pandemic.
The curious thing is that the new rate gets locked in, even when the initial cause dissipates.
Between 1980 and 2007, U.S. birth rates stayed within a narrow range of roughly 65–70 births per 1,000 women aged 15–44. But between 2007 and 2020, birth rates plummeted by about 20%, falling to 55.8 last year. [Axios]
Triggered by the GFC, but did not bounce back. Collectively it seems that we are continuously wanting fewer children, and short chocks to the system cause adjustments in our behaviour, that were always on the verge of happening.