The report, by the New York City-based Guttmacher Institute, a non-profit that has been tracking abortion since 1974, shows that medical abortions using the drug RU-486 are increasing as a percentage of all abortions. Guttmacher's periodic reports are based on surveys of abortion providers.

This report uses statistics from 2005, the most recent year available. It is being released just before Tuesday's 35th anniversary of the Roe v. Wade Supreme Court decision legalizing abortion.

The report finds:

•Medical abortion, which uses RU-486, known as mifepristone and sold as Mifeprex to induce abortion, is rising. Medical abortions more than doubled since federal approval of the non-surgical method in 2000, from 6% of all abortions that year to 13% in 2005.

•The number of abortions, which peaked in 1990 with 1.6 million, dropped to an estimated 1.2 million in 2005. That's 8% fewer than 2000.

•The abortion rate, which is the proportion of pregnancies that end in abortion, dropped by 9% to 19.4 abortions per 1,000 women ages 15-44, in 2005. The peak — 29.3 abortions per 1,000 women — was in both 1980 and 1981. In 1974, a year after abortion became legal, the rate was 19.3.

•The number of abortion providers in the USA also dropped to 1,787, representing a 2% decline from 2000.

•Just over one in five pregnancies in 2005 (22%) resulted in abortion, down from one in four pregnancies (25%) in 2000.

Randall O'Bannon, director of education and research for the National Right to Life Committee based in Washington, DC., says the decline is a sign of progress but more needs to be done to reduce it further.

"I'm pleased that it's going down," he says. "We were at 1.6 million in 1990. So, for it to drop to 1.2 million is a significant decline."

Carole Joffe, a sociology professor at the University of California, Berkeley, who has been studying abortion since 1976, says the abortion pill didn't increase the number of abortions nor did it necessarily increase access.

"There was a lot of naiveté that immediately many primary care doctors would start offering this and dramatically increase access to abortion," Joffe says. "In retrospect, that was very naïve and didn't happen."

(After the article's publication, Joffe said the above quotation attributed to her in the online edition of USA TODAY was incomplete; she notes she also stated that "one of the most notable findings of this new study was precisely the steady growth over time in the number of providers offering mifepristone and the overall number of abortions performed using this method.")

Guttmacher's data is considered the more thorough of the two entities that monitor national data. The other source is the Centers for Disease Control and Prevention, which relies on voluntary reporting by state health departments. Guttmacher supports abortion rights.

Tracy Weitz, director of the Advancing New Standards in Reproductive Health program at the University of California, San Francisco, says data from both the CDC and Guttmacher have limitations. She says CDC can only report what the states provide and the data is not reported uniformly or consistently and misses some states. Guttmacher's provider survey looks at the facilities where abortions are provided but doesn't show whether a facility has one doctor or many, which makes it difficult to determine the number of individuals performing abortions.

CDC's information, most recently released in November, does not include statistics from California, West Virginia and New Hampshire. CDC spokeswoman Reneé Brown-Bryant says the 49 reporting areas include New York City, Washington, D.C., and 47 states.

"We used to make estimates for the ones we didn't have, based on population data, but we stopped because we didn't have enough data to do valid projections," she says.

Rachel Jones, a senior research associate at Guttmacher, says California accounts for one in five abortions in the USA.

Those who study abortion aren't sure why there's been a continued decline, but some, such as economics professor Ted Joyce of Baruch College in New York City, say it's likely a combination of factors such as better contraception, greater awareness about teen pregnancy and even welfare reform.