The American birthrate is quite low among reasonably well-off whites - in other words people who correspond to Europeans. The reason the overall birthrate is so high in the US is because of very large numbers of desperately poor whites and of course even larger numbers of extremely disadvantaged minority people.
I would assume the main reason is a more favorable system of tax breaks meself...
That certainly is the main reason France has a much healthier birth rate than Germany.
And then again, there's also a somewhat larger contingent of people who are absolutely nuts and prefer to show this by breeding like rabbits. How many kids does Rick Renzi have again? 16?