In many western countries the reason why people aren't having as many kids as before is because it's too expensive, and because housing is too expensive. You aren't going to have children, if you're still living in your parent's basement, or sharing an apt with roommates.

Childcare in the US, for example, is outrageously expensive. If you aren't lucky enough to have a grandparent or other relative do it for free, or for nominal compensation, your childcare cost is prohibitively expensive. In many cases it's so expensive, that it's more cost effective for the parent with lower salary to quit and stay home. Hopefully programs like the free childcare New York City is offering become more common.