In raw terms, the number of abortions fell below one million a year for the first time since 1975, according to Guttmacher.

The numbers come from a survey of all known abortion providers that Guttmacher conducts every few years. The researchers supplemented the survey results with data from state health agencies to arrive at their estimates.

The researchers identify two main factors driving down the incidence of abortion. The first is a drop in the rate of unintended pregnancy, driven primarily by an increase in the use of long-term contraceptive measures, like IUDs, that are highly effective. The Guttmacher report notes that the use of these contraceptives increased by 36 percent between 2009 and 2012, and even higher among younger and lower-income women, two groups who are particularly at risk for unplanned pregnancy.

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More restrictive state-level abortion laws may also be playing a role. Numerous states saw a decline in the number of abortion clinics from 2011 to 2014, or enacted other restrictions on the practice, including waiting periods or bans on certain types of abortion.

However, the report points out the relationship between abortion law and abortion in practice is not always clear cut. For instance, several states in the Northeast saw an increase in the number of abortion providers from 2011 to 2014, but the abortion rate still fell by 11 percent in that region

On the other hand, the incidence of abortion increased slightly in a small number of states, including Kansas, Mississippi and North Carolina, that have enacted some of the tightest restrictions on abortion.

Overall “it is unclear whether the most recent decline in abortion is due to fewer women’s having unintended pregnancies, more women’s being unable to access abortion services or some combination of these dynamics,” the report concludes.