Roughly a fourth of American women getting early abortions last year did so with drugs rather than surgery, statistics show, as a new study reported improved safety using the so-called abortion pill.

Some experts predict the percentage of such "medical abortions," which offer more privacy than surgical termination at a clinic or hospital, will rise even more after the new study.

The research, done at Planned Parenthood clinics across the country, shows that a new way of giving pills to induce abortion virtually eliminated the risk for a rare but dangerous infection.

"This is the first really huge documentation of how safe and effective medical abortion is," said Beverly Winikoff, a professor of family health and population at Columbia University. "The technology is very good and very well used in this country, and probably will be used more and more."

Two pills are used to induce an abortion. The primary drug, Mifeprex, was first approved in the United States in 2000. Use has risen steadily, to 184,000 women last year, even though manufacturer Danco Laboratories L.L.C. hasn't promoted it and the drug can only be obtained at a clinic or doctor's office.

Medical abortions now account for about a quarter of early abortions, said company spokeswoman Abby Long. At Planned Parenthood, the biggest provider of medical abortions, they amount to 32 percent of early terminations.

The group's study analyzed medical abortions at Planned Parenthood centers between 2005 and mid-2008 - about 228,000 cases. It found that the abortion pill was 98.5 percent effective and that changes in how the drugs were given reduced risk of a serious infection from barely 1 in 1,000 cases to 0.06 in 1,000. The results are reported in today's New England Journal of Medicine.

The procedure, which works during the first nine weeks of pregnancy, involves swallowing Mifeprex, known chemically as mifepristone, at a doctor's office. Originally called RU-486, the pill causes an embryo to detach from the uterine wall. A second pill, misoprostol, is used 24 to 48 hours later to cause contractions and push the embryo out of the uterus.

These drugs are different from Plan B, which is taken within a couple of days after sex to prevent pregnancy.

Originally, the procedure involved inserting the misoprostol pill into the vagina, where the it was absorbed. But by the end of 2005, four American women and one Canadian had died of a rare bacterial infection, spurring concern among providers and criticism by abortion opponents.

So in April 2006, Planned Parenthood told its 300 clinics offering the procedure to have patients put the misoprostol pill in their mouth and let it dissolve. By the study's last six months, serious infections had declined to one-16th of the original rate.

"I think that providers are going to be pretty impressed with the data," said Beth Jordan, medical director of the Association of Reproductive Health Professionals, which plans to highlight the findings to doctors, nurse practitioners, and pharmacists at its annual meeting in September.

Given that 87 percent of counties have no abortion provider, Jordan said the findings might encourage some primary-care doctors to offer abortion using the pills. "But I don't think it's going to open the floodgates," she said.