As the standard of living rises, the birthrate initially increases. This was the boomer years.
Then birthrate falls because an industrialized society requires more resources per child. Eventually you reach a period of stagnation when the resources available match the number of children born.
Then in all capatilist societies, the standard of living declines which triggers the birthrate to free fall.
https://www.macrotrends.net/global-metrics/countries/jpn/japan/birth-rate
Japan peaked in the 1940's, stagnanted in the '60-'70s and has been in free fall ever since.
https://www.macrotrends.net/global-metrics/countries/kor/south-korea/birth-rate
South Korea peaked in the 1950's, stagnated in the '80s and 90's and has been in free fall since.
https://www.macrotrends.net/global-metrics/countries/usa/united-states/birth-rate
The United States peaked in the 40's-50's. Stagnanted from the 1970's to early 2000's (bouyed by 1st generation immigration) and is now in free fall (due to immigration idiocracy).
https://www.macrotrends.net/global-metrics/countries/fra/france/birth-rate
France followed the U.S. due with an extended period of stagnation due to 1st generation immigrants. It also has had a much slower decline in the standard of living versus the other countries which has bouyed up the birthrate.