The study, conducted by Josiane Warzawski, MD and her Institut National de la Sante et de la Recherche Medicale (INSERM) colleagues throughout France, enrolled 5,271 mother-child pairs with deliveries between 1997 and 2004. Overall, 67 (1.3 percent) infants born to HIV-positive women were infected with the virus.

Viral load, at the time of delivery, was found to be a major transmission risk factor. Among the 364 women who delivered with viral loads above 10,000 copies, the transmission rate was 6.6 percent, compared with a rate of 0.6 percent among the 2,856 mothers who delivered with viral loads below 400 copies. The transmission rate among mothers with viral loads less than 50 copies at the time of delivery was 0.4 percent.

Another key factor was the duration of HIV drug treatment during pregnancy. There were no transmissions among women who were receiving antiretroviral therapy when their pregnancy was first documented and remained on treatment. Among HIV-positive pregnant women who started treatment during the first or second trimester, the transmission rate was approximately 1 percent. And among those who started during the third trimester, the transmission rate was 3.6 percent. The authors noted that the five mothers who transmitted HIV with viral loads below 50 copies at the time of delivery started treatment late in their pregnancies, between weeks 32 and 33.

Infants born premature were also at a higher risk for infection. According to the INSERM report, the transmission rate was six times higher for babies born before 33 weeks (6.6 percent) than for full-term infants (1 percent).

Dr. Warzawski’s group concluded that the “most effective means available to obtain such a control of viral replication is the use of triple combination therapy. Furthermore, our findings strongly suggest that antiretroviral therapy should be started relatively early, at the latest by 28 weeks, to obtain maximum efficacy.”