The researchers also found that pregnant women with the highest levels of HIV were one-third less likely to transmit the virus if they were treated with the anti-retroviral drug AZT than those who were not.
The results were published in Thursday's edition of the New England Journal of Medicine.
The researchers concluded that aggressive anti-retroviral therapy is probably the best way to lower the risk that babies will be born with AIDS. However, they warned that the benefits must be weighed against the possible long-term effects of the drug, which are still unknown.
One study, led by Dr. Lynne Mofenson of the National Institutes of Health, looked at women who were treated with AZT while pregnant and whose babies also were treated with the drug for several weeks after birth.
Among the 84 mothers in the study who had undetectable levels of the HIV at 20 to 30 weeks of pregnancy, none of the babies tested positive for HIV.
The second study, led by Dr. Patricia Garcia of Northwestern University, looked at 552 pregnant women from 1990 to 1995.
It found that among women with the highest levels of HIV, representing the most advanced cases of AIDS, 20 percent of those who had been treated with AZT transmitted the virus to their babies compared to a 63 percent transmission rate among those not treated with AZT.
In countries that cannot afford AZT treatment, about one-fourth of all babies born to HIV-positive mothers get the virus. Since AZT therapy became standard in the United States, that rate has dropped to 9 percent.