Increasing the level of education for women. Increasing the access to pre-natal care for women. Decreasing the poverty level generally and specifically for young families. Moving more people out of agricultural work.
Those things are highly correlated with lowered birth rates, though I suspect the last one is probably not applicable to the US.
The birth rate in the US is 1.64 children per woman. It’s already a significantly shrinking population, replacement rate is around 2.1 children per woman. The US only maintains its population through immigration.
Those things are highly correlated with lowered birth rates, though I suspect the last one is probably not applicable to the US.