One thing that I've noticed in the US is that Roman Catholic families and Orthodox Christian families are about the same in numbers of children.
There are exceptions, of course. My dad was one of 8 children and my mother was one of nine (no, that's not a name you Star Trek geeks!). However, you don't see many families that large any more.
We are no longer and egrarian society. Children are not needed to help feed, support the family hence there is not the need to continually add hands to support the family unit...that and economics for many...
I'm not saying that I think it's a good thing, but it's pretty much the way it is here...other countries where they still support themselves by their families production of food is an entirely different story...