Every since the start of the industrial revolution, children became an economic burden instead of a benefit. Once man power was replaced by machines, it stopped making sense to have so many kids and the total fertility rate started to decline. The data is sparse prior to 1950, which is coincidentally when there was a huge global post war baby boom, but visit https://ourworldindata.org/fertility-rate and scroll down to births per woman and look at someplace like Sweden. It was already going down! Prior to modernity and its ills. TFR was higher when people felt like they had to have kids to survive a harsh world.