The same thing happened (declining fertility rates) between WW1 and WW2, which I found interesting. Also a time of great wealth inequality and economic dislocation.
Another thing I got from the same book (Dark Continent) was that pre-Nazi Germany was basically being run by the equivalent of executive orders due to political polarisation and deadlock in the Parliament. Definite vibes of modern US in that statement.
https://en.wikipedia.org/wiki/Dark_Continent:_Europe%27s_Twe...
I don't know if the same was true in Rome, but indirect evidence would suggest it may well have been. Here [1] is a selection of some of the laws that were created to motivate fertility. They definitely target what were probably higher class citizens - concubinage was legally recognized, inheritance was one of the main targets of penalties, ranking of politicians was determined by their number of children, and so on. There was even apparently some angle shoot where people tried to marry very underage girls to avoid the penalties against unmarried men while also having a legal justification for having no children 'yet'.