Abortions in the U.S. resumed their downward trend between 2008 and 2011, according to a new study. But its authors say the recent surge of state laws intended to restrict the procedure is likely not the reason.

Both the abortion rate and the number of abortions (just under 1.1 million) fell 13 percent, according to the Guttmacher Institute, the reproductive health think tank that's been doing the periodic survey of abortion providers since the 1970s. The 2011 abortion rate of 16.9 per 1,000 women ages 15-44 is the lowest since 1973, the year the Supreme Court legalized abortion nationwide.

The survey also found a continuing increase in the use of medication abortions, as opposed to surgical ones. An estimated 23 percent of all abortions performed outside of hospitals were nonsurgical, the study found — up from 17 percent in 2008.

But the authors said they found no evidence that the passage of hundreds of state abortion restrictions had any impact on the latest decline. "While most of the new laws were enacted in states in the Midwest and the South, abortion incidence declined in all regions," the study noted.

And some states that are generally supportive of abortion rights, including those allowing Medicaid payment for abortions for poor women, "experienced declines in their abortion rates comparable to, and sometimes greater than, the national decline," the study said. It cited California, New Jersey and New York as examples.

The authors suggest two more likely possibilities.

One was the economy, which was in recession or slow recovery during the survey period. Evidence for that comes from the fact that the birthrate also fell — by 9 percent — between 2008 and 2011, meaning fewer women got pregnant overall.

"Presumably then, more women and couples were making conscious decisions to avoid pregnancy and so resumed or continued using contraceptives," the authors noted. "This strategy would be expected to have a bigger impact on the rate of intended pregnancies than on the abortion rate, but could also have averted the 5 percent of abortions that followed intended pregnancies."

The other likely reason for the abortion decline is the increase in the use of long-acting contraceptives like the IUD, which are much more effective in preventing pregnancy than condoms or other nonprescription methods. Long-acting-method use in publicly funded clinics increased from 4 percent to 11 percent during the study period.

One wild card in the study is a possible increase in the use of the ulcer drug misoprostol to terminate pregnancy. While not approved for use alone (it is normally used in combination with the approved medication abortion drug Mifepristone), an unknown number of women are using it by itself, and it can terminate an early pregnancy.

The study suggests that if more women are turning to off-label misoprostol use to terminate their pregnancies "outside clinical settings," then "our estimate of the number of abortions is artificially low."

Meanwhile, anti-abortion groups are taking exception to the study's conclusions.

Charmaine Yoest, president and CEO of Americans United for Life, called the report "an abortion industry propaganda piece short on data and long on strained conclusions," noting that all of the reporting of data is voluntary.

Copyright 2021 NPR. To see more, visit https://www.npr.org.