Abortions have been falling steadily since their peak in 1990. Kevin Drum posts the graphic at left, which I gratefully stole from him, and links to an article trying to explain the decline:
Abortion rights advocates suggested women may be avoiding unwanted pregnancies, thanks in part to the morning-after pill, emergency contraception that is sold without a prescription to women 18 and older.
Conservatives, by contrast, [focus on] laws in more than 30 states mandating counseling before an abortion.
Kevin does an able job of explaining why the conservative argument is nonsense: the pregnancy rate has dropped, and the abortion rate has dropped in states that don't have counseling requirements. He doesn't do quite as good a job at explaining why the abortion-rights activist explanation is also wrong--or at least, not backed up by the available evidence. The abortion rate has been falling pretty steadily for almost 20 years, but the morning after pill, though it has been widely known about since the 1980s, has only been legal in the United States since the late 1990s. Neither of those developments matches up with the observed drop in abortions very well. Nor is better sex ed a very good explanation; I'm aware of no revolution in sex-ed that occurred in the late eighties.
The best explanation may be AIDS; unprotected sex is riskier, so people are having much less of it. The fall roughly matches up with the widespread change towards perceiving AIDS as a heterosexual problem. But that's an offhand guess. We really don't know why the abortion rate has fallen, though I'm sure we're all glad it has.