If the whole world ends up "first world", population won't grow as quickly. In a harsh country, it may be necessary to have as many children as possible, because of high infant mortality, the need for elders to be supported by their children due to a lack of social security, or lots of other reasons. If everyone ends up being "westernised" (and it will probably happen eventually) then birthrates will probably drop.

That's my personal lunatic theory on the topic, anyway...