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 from 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 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.”

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 record-setting period that saw the enactment 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 the procedure 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, including Tobias’s, reject the Guttmacher Institute’s conclusion that the decrease is 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. For example, 39 states require parental notification or consent for a minor to get an abortion.

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 even 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.”

Guttmacher researchers said it is unlikely that previous laws had an impact, because they examined the abortion rate in more liberal states that did not enact such laws prior to 2011 and found no difference in the trend.

The study released Monday shows that, after a plateau from 2005 to 2008, the long-term decline in the abortion rate has resumed. The rate has dropped significantly from its all-time high in 1981, when there were roughly 30 abortions for every 1,000 women of reproductive age. The overall number of abortions also fell 13 percent from 2008 to nearly 1.1 million in 2011, the study said.

The results echo a report last year from the Centers for Disease Control and Prevention, which also found a decline in the abortion rate after a plateau. That report, which used a different methodology, pegged the abortion rate in 2010 as 14.6 abortions per 1,000 women of reproductive age.

The numbers of abortions are not easily sussed out, because providers are not required to report them. The CDC draws from state health department reports, and its data are incomplete. For example, California — responsible for an estimated 17 percent of abortions nationwide — does not gather data on the number of abortions.

Guttmacher approaches the task differently, conducting a periodic census by mailing a questionnaire to all known abortion providers. They follow up over the phone with those who do not respond and use health-department and other data to fill any remaining gaps. The report is set to be published in the March issue of the peer-reviewed journal Perspectives on Sexual and Reproductive Health.

Experts cautioned that the numbers documented by Guttmacher in the immediate aftermath of Roe v. Wade may be particularly shaky. Many abortions were still taking place underground and off the books at that time.

Monday’s report showed a shift in women’s preferred method of abortion. Researchers found that nearly one in four of all non-hospital abortions were a result of the abortion pill, up from 17 percent in 2008. The total number of abortion providers declined 4 percent over the same period.

Six states experienced no change or an increase in their abortion rates: Alaska, Maryland, Montana, New Hampshire, West Virginia and Wyoming. Researchers did not explain why those states defied the overall trend.