“Premodern?” The US fertility rate fell from 3.65 in 1960 to below replacement in 1972. In that time, the labor force participation rate of married women went from 32% to 42%. Then it’s been pretty stable since then between 1.8-2.1, even as the labor force participation rate for married women went from 42% to 67%. We are talking about much more recent social trends at issue.