I can't speak to places outside of the Western Anglosphere, because I simply don't have any information about them—but within it, it's abundantly clear (and there have been some recent studies bearing this out) that a major cause of reduced birth rates is lack of economic opportunity among the non-capital-owning classes.

I 100% guarantee you that if we implemented a full UBI today—one that would pay something close to the median individual income, and even if it were only for adults (and thus you got no "bonus" for having extra kids)—once the initial chaos settled down, you'd see those birth rates go up quite a bit. So many people are waiting to have kids until they're financially stable enough...and then they never become so.

I know that one of the big reasons my wife and I didn't have kids in our 20s was because we were concerned about our financial stability (and frankly, we were better off than most of the people that age today—by quite a ways, since we were homeowners).