The study did not examine the reasons for the drop. But the authors suggested that one factor was greater reliance on new kinds of birth control, including intra-uterine devices such as Mirena, which can last for years and are not susceptible to user error like daily pills or condoms.

They also noted the economy as a contributing factor, because people tend to adhere more strictly to their birth control during tough economic times. But they did not credit the recent wave of state laws restricting access to abortion, because most of those took effect in 2011 or later.

Those restrictions will surely have an impact on the numbers going forward, said Rachel K. Jones, a senior researcher at Guttmacher and lead researcher on the paper.

“If the abortion rate continues to drop, we can’t assume it’s all due to positive factors” such as better adherence to contraceptives, she said, calling the laws passed in 22 states “onerous.”