It's pretty clear that the major driver is education and freedom of career choice for women. The UN has lots of data supporting that. Industrialization and changing attitudes toward children have also played a role.
My paternal grandfather was a farmer. He had 20 children who survived to adulthood. By two wives, the second having been the nursemaid. As I understand it, all of the children worked. Basically as soon as they could walk. Girls focused on childcare and housework, and boys on farm work. If they'd been living in a city, the boys would have had factory jobs.
So anyway, that's how you raise many children. That or be wealthy, of course. But current expectations, social norms, and laws make that impossible. Except perhaps for farm families, where substantial child labor remains legal.
Education comes with higher life standards. You want your kids to have their own rooms instead of whole family living in single-room shack, education for them etc.
"Freedom of career" for women is more like freedom to have to work. It's hard to raise a family on a single salary. Previously, a woman would work at home and husband would work outside. Now both of them have to work outside of the house to pay for appliances that do stuff back at home.
IMO one of the top issues is housing cost. Increasing specialisation, both parents having to work and urbanisation means people are crammed into more and more condensed cities. Space comes at expense. Children need space. Cheaper space out of the city means more transport costs and/or less job opportunities and likely lower wage. All in all, children is damn expensive.
Yes, it's true that (at least in the US) greater career choice for women has come with declining real wages. So that now, both partners need to work. Some argue that effectively doubling the workforce has played a major role. Or that people just expect to have more stuff. But the power of unions to maintain livable wages has eroded over the same time frame. And maybe it's all connected.
My paternal grandfather was a farmer. He had 20 children who survived to adulthood. By two wives, the second having been the nursemaid. As I understand it, all of the children worked. Basically as soon as they could walk. Girls focused on childcare and housework, and boys on farm work. If they'd been living in a city, the boys would have had factory jobs.
So anyway, that's how you raise many children. That or be wealthy, of course. But current expectations, social norms, and laws make that impossible. Except perhaps for farm families, where substantial child labor remains legal.