Why is this happening?

Some conjecture that improved access to and use of birth control may be the reason, but there’s scant evidence for this. At any given moment nearly 40 percent of women are using no birth-control method at all. Almost half of all American pregnancies are unintended.

Abortion rates are declining because more and more of these unintended pregnancies are being carried to term. Again, some conjecture that women are deciding to carry their unintended pregnancies because they are denied access to abortion. There isn’t much evidence for this proposition either. While access to abortion has been curtailed by conservative state legislatures since the 2010 election, most of the decline in abortion incidence occurred much earlier. Even post-2010, abortion remains readily available in all major U.S. population centers. Although there are abortion clinics in only 11 percent of U.S. counties, those counties are home to 62 percent of American women, and many of the remaining women live in counties adjoining those in which clinics are found. There is not a single state—not Mississippi, not South Carolina, not Wyoming—that has no abortion clinic at all.