An interesting argument I've heard wrt to "high" western birthrate countries is that it has more to do with the societal roles of women. In both the United States and Sweden, women are generally told they can have both a career and become mothers, and while this view is not universally held in either country, it's a widespread belief. By contrast, in western nations with lower birthrates, the social views tilt towards women being told they can have a career OR become a mother, and that trying to be both is detrimental.

What tends to happen in nations when motherhood becomes an either-or proposition is, women opt to either forgo childbearing or chose to only have 1 child and then continue on with their career track. Whereas in countries with a broader view of motherhood, women who want both aren't guilted by society to stop at 1 or, and likewise fewer women face the choice of forgoing or limiting motherhood to pursue their career ambitions.