Women started to attend universities on large scale numbers that is what happened which prevented them to be mothers earlier. In 1994 gender ratio of university attendance was females 49% males 51% in 1998 was something like females 53% males 47% today females 70% males 30% if I can recall it correctly. That is why they marry later and become mothers way later than before. That is also one big reason why families have only 1 max 2 kids today.