WASHINGTON — The abortion rate in the United States dropped to its lowest point since the Supreme Court legalized the procedure in all 50 states, according to a study suggesting that new, long-acting contraceptive methods are having a significant impact in reducing unwanted pregnancies.

There were fewer than 17 abortions for every 1,000 women in 2011, the latest year for which figures were available, according a paper published Monday by the Guttmacher Institute, a pro-abortion-rights think tank. That is down 13 percent from 2008 and a little higher than the rate in 1973, when the Supreme Court handed down its landmark Roe v. Wade decision.

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 intrauterine devices such as Mirena, which can last for years and are not susceptible to user error as are daily pills and condoms.

They also noted the economy as a contributing factor, because people tend to adhere more strictly to 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 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.”

The report comes as tensions intensify in the long-simmering debate over abortion and contraception. Religious groups are locked in a closely watched battle with the Obama administration over new rules that require employers to offer birth control free of charge as part of their health insurance benefit packages. The Supreme Court will decide this year whether employers with religious objections may opt out of those rules.

State legislatures are preparing to push through another raft of restrictive laws, after a period that saw the enactment of dozens of new regulations that critics say will impede women’s access to abortion.

The new laws include requirements that women undergo ultrasounds before obtaining abortions, as well as licensing and inspection requirements for abortion providers.

Nine states banned abortions after 20 weeks of pregnancy, part of a national effort by abortion opponents to force the Supreme Court to revisit the legality of abortion.

The political clashes, now and over the years, have overshadowed a trend cheered, albeit cautiously, by both sides of the issue: The number of abortions in the United States has been decreasing.

“We are extremely happy that the abortion numbers are going down and continue to be declining over the years,” said Carol Tobias, president of National Right to Life, a prominent antiabortion group.

Such groups reject the Guttmacher Institute’s conclusion that the decrease was not related to state regulations restricting access to the procedure, because while the major surge in new laws came in 2011, some laws came earlier.

They say the graphic conversation in the 1990s around the procedure they call “late-term” abortion contributed to a greater awareness of, and horror over, how abortions are performed. And they credit new technologies that allow people to better observe what happens in the womb at the earliest stages of pregnancy.

“This is a post-sonogram generation,” said Charmaine Yoest, president of Americans United for Life, the group behind many of the new state limits on abortions. “There is increased awareness throughout our culture of the moral weight of the unborn baby, and that’s a good thing.”