Fertility is falling in Latin America too. The export of birth control and abortion is taking its toll there, though slower than here and in Europe and Asia. That is because traditionally they’ve valued large families, but that is changing. Only in Africa have they (so far)3 been more successfully resisting it.
Traditionally Americans valued large families when children added to family support at an early age, 8-10 years old or so and not so many of them lived to be adults. The biggest spur to population increase has been the spread of Western medicine to the poor countries before they prosper past the stage of children being net assets.