The economic conditions always feels weird. Aren't birth rates higher in places with worse conditions? And weren't they higher when conditions were worse?
There’s a few dynamics at different scales. Worse conditions are associated with subsistence farming and manual labor where children are more immediately valuable. Child care tends to be easier in more rural communities. Birth control access is lower. Land and housing tends to be cheaper.
Child mortality is much higher in worse conditions as well, and having more children the increases chances that some survive.