Less education (including financial and reproductive education) correlates with lower financial status pretty heavily. It's probably more directly related to education than wealth, but education tends to also be correlated with wealth.

Wealthier people are more likely to be more educated, and likely defer on having kids until it makes financial sense, while also having less kids due to resulting loss of time and wanting to maintain high financial security.

I'd imagine poorer people are more willing to ignore the financial effects of having a kid while younger (or of having multiple kids), or not even realize the financial effects, due to overall less education. Especially if they see no hope in increasing their financial status in the future. Plus of course less access to birth control etc.