Generally the less money you make the more kids you have. It's really a question of prioritization. People say they're holding off on kids for X or Y reason but I think this is more of an expressed vs revealed preferences situation. They would rather chase material wealth for themselves than have kids, and to be clear I'm not judging just observing. Through most of human history mud huts weren't a blocker to having kids.
https://www.statista.com/statistics/241530/birth-rate-by-fam...