Birth rates will go up because they are inheritable and because exponential growth always wins. The USA will be Amish.
It's only a change in environment that can knock that down a bit, but exponential growth still wins. Birth control cut the birth rate for most people, but not for all. The resistant sub-populations are still on exponential growth even as the other sub-populations go through a population crash.
It's as certain as math. Exponential growth wins.