Anti-abortion activists contend that falling abortion numbers are evidence that their efforts are having an impact, even as reproductive-rights advocates say otherwise. Katherine Frey/The Washington Post/Getty Images

The number of abortions performed in the United States has dropped to the lowest level in 40 years, according to a study by the Guttmacher Institute, which supports legal access to abortion. The study suggests the decline is due more to the increased use of contraception than increased restrictions on access to abortions.

In 2011 an estimated 16.9 abortions were carried out per 1,000 women ages 15 to 44 — that’s 1.1 million in absolute terms.

It was the lowest number since 1973, when the figure stood at 16.3 per 1,000, according to the report, "Abortion Incidence and Service Availability in the United States, 2011," which will appear in the March 2014 issue of Perspectives on Sexual and Reproductive Health.

Guttmacher's figures are of interest on both sides of the abortion debate because they are more recent and in some ways more comprehensive than abortion statistics compiled by the federal Centers for Disease Control and Prevention.

Carol Tobias, president of the National Right to Life Committee, described the overall drop in abortion numbers as evidence that the anti-abortion movement's lobbying and legislative efforts were having an impact.

"It shows that women are rejecting the idea of abortion as the answer to an unexpected pregnancy," she said.

From 2008 to 2011, the abortion rate fell by 13 percent, with more procedures were performed earlier in pregnancy.

Rachel Jones, the lead author of the report, said there appeared to be no link to a decline in the number of abortion providers.

"With abortion rates falling in almost all states, our study did not find evidence that the national decline in abortions during this period was the result of new state abortion restrictions. We also found no evidence that the decline was linked to a drop in the number of abortion providers during this period," she said.

According to the report, the total number of providers dropped by 4 percent, to 1,720, from 2008 to 2011, and the number of abortion clinics declined by just 1 percent, to 839.

The drop in abortions likely stemmed from a steep national decline in overall pregnancy and birth rates, Jones said.

"Contraceptive use improved during this period, as more women and couples were using highly effective long-acting reversible contraceptive methods," she said. "Moreover, the recent recession led many women and couples to want to avoid or delay pregnancy and childbearing."

While the overall abortion rate declined, the proportion of abortions entailing early medication procedures continued to increase. According to Guttmacher, about 239,400 abortions of this type were performed in 2011, representing 23 percent of all nonhospital abortions, an increase from 17 percent in 2008.

Beginning in 2011, state efforts to restrict abortion have surged, according to the report. States enacted 205 abortion restrictions from 2011 to 2013 — more than in the entire previous decade combined. Texas and Arizona, for example, are among the states that in recent years passed legislation to ban abortions after 20 weeks of pregnancy. In January the Supreme Court declined to hear a case on Arizona’s abortion restrictions, and the House of Representatives on Tuesday voted to bar federal subsidies for Americans signing up for health-insurance plans that cover abortion.

Americans United for Life, another anti-abortion group active in the efforts to pass restrictive state laws, said Guttmacher's numbers should be viewed skeptically because they are based on voluntary reporting by abortion providers.

"It is impossible really to know the true abortion rate," said Americans United for Life's president, Charmaine Yoest.

The report marked the 16th time since 1973, when abortion was legalized nationwide, that Guttmacher has attempted to survey all known abortion providers in the U.S. However, a section of the new report acknowledges that some abortions might not be tallied.

The highest abortion rates were in New York, Maryland, the District of Columbia, Delaware and New Jersey; the lowest were in Wyoming, Mississippi, South Dakota, Kentucky and Missouri. Guttmacher said many women in Wyoming and Mississippi, where providers are scarce, go out of state to get abortions.

Al Jazeera and wire services