I've read plenty researches: main factor is healthcare. It's a natural a better healthcare - leads to lower child and women death rate. That's why people are having less child and they become more valuable.

100 years ago there's NO statistic for child under 13yo death rate. Mostly because it was TOO high around 50-60% children are dying before they get ready for marriage.

My grand-grand-ma told me she had 8 brothers and sisters and only 2 survive: she and her brother. In that time it was reasonable to bake kids in enormous numbers in hope some of them are won't die. Then healthcare changed everything.