The New York Times has a go at the question of why America's birthrate has remained stable while Europe's decreases:
But one other factor affecting the higher U.S. birthrate stands out in the minds of many observers. “There’s much less flexibility in the European system,” Haub says. “In Europe, both the society and the job market are more rigid.” There may be little state subsidy for child care in the U.S., and there is certainly nothing like the warm governmental nest that Norway feathers for fledgling families, but the American system seems to make up for it in other ways. As Hans-Peter Kohler of the University of Pennsylvania writes: “In general, women are deterred from having children when the economic cost — in the form of lower lifetime wages — is too high. Compared to other high-income countries, this cost is diminished by an American labor market that allows more flexible work hours and makes it easier to leave and then re-enter the labor force.” An American woman might choose to suspend her career for three or five years to raise a family, expecting to be able to resume working; that happens far less easily in Europe.
So there would seem to be two models for achieving higher fertility: the neosocialist Scandinavian system and the laissez-faire American one. Aassve put it to me this way: “You might say that in order to promote fertility, your society needs to be generous or flexible. The U.S. isn’t very generous, but it is flexible.
As for Germany and Austria:
They share many of the same characteristics of other Western European countries with regard to forces affecting family life, but in addition childlessness is peculiarly high in these countries, and has been for some time. A 2002 study found that 27.8 percent of German women born in 1960 were childless, a rate far higher than in any other European country. (The rate in France, for example, was 10.7.) When European women age 18 to 34 were asked in another study to state their ideal number of children, 16.6 percent of those in Germany and 12.6 percent in Austria answered “none.” (In Italy, by comparison, this figure was 3.8 percent.) The main reason seems to be a basic change in attitudes on the part of some women as to their “natural” role. According to Nikolai Botev, population and development adviser at the United Nations Population Fund, many observers have been surprised to find that in recent years “childlessness emerges as an ideal lifestyle.” No one has yet figured out why some countries are more predisposed to childlessness than others.
The question lurking in the background: Have these German/Austrian women given up on child-bearing because they genuinely have no interest in raising children? Or because it's just too hard to combine career and family, and you might as well not want what you don't think you can get?
Purely anecdotally, I know a few German women who say they're in group number one, but when you question them a bit more closely, they sound more and more like they're actually in group two...

The source: