The drug nevirapine prevents the spread of the AIDS virus from mother to child time after time, a new study suggests, challenging earlier findings.

The new research presented Wednesday at a scientific meeting in Denver found that in Ugandan women who received the drug during a first pregnancy, HIV transmission was prevented during second pregnancies as well.

The research may ease concerns raised in previous studies that HIV develops resistance to the drug, said Dr. Michael Thigpen, a medical epidemiologist with the U.S. Centers for Disease Control and Prevention.

"Based on these findings, we believe nevirapine in repeat pregnancies remains an effective option in these resource-limited countries," said Thigpen, who is part of the research team.

The study looked at 198 women treated in 2004 and 2005 at a hospital in Kampala, Uganda.

Nevirapine is an inexpensive and easy-to-take medication that has become a mainstay in the effort to prevent mother-to-child HIV transmission in poor countries. Proponents say the drug cuts the transmission risk in half.

Mothers receive a pill when they go into labor, and their newborns get the medication in a syrup within 72 hours of birth.

The drug came into question in earlier studies done in South Africa and Uganda, which found that 20 percent to 40 percent of HIV-infected women developed resistance to nevirapine after taking one dose to protect their newborns from getting the infection.

The new research found that the HIV infection rate was 14.6 percent for babies born to nevirapine-treated women who also took the drug during a previous pregnancy. The rate was 17.6 percent for nevirapine-treated women who were not given the drug during an earlier pregnancy.

The research is good news, said Mark Isaac, a vice president for the Elizabeth Glaser Pediatric AIDS Foundation, which funds HIV/AIDS research and worldwide treatment programs.

A study of multiple pregnancies in South Africa and Ivory Coast, also announced Wednesday, produced similar results. Three other studies showed low risks of nevirapine resistance in women who took the medicine more than a year after the initial dose.

"It's fair to say we're breathing a sigh of relief," Isaac said.

Thigpen said there are different subtypes of HIV, and one that circulates in southern Africa appears especially resistant to nevirapine.

"In Uganda, there are a couple of different subtypes. That (fact) may limit our ability to associate these findings with other areas of Africa," Thigpen said, referring to his study's findings.