[ajsnigrutin]

Those are not directly related (although in some way they are).

Lack of education, contraceptives and social nets means that women in congo will have more children on average. If you get them all that and a stable pension after (so they won't need kids to keep them alive when old) the numbers of kids will fall.

On the other hand, in the "rich" western countries, no matter how high the income number is (compared to congo), that doesn't mean that a young couple can actually afford a home large enough to have a kid or two (or more), childcare and all the other kid-related costs.