Eighty-five percent of the cumulative totals of children younger than thirteen who have been reported to have HIV infection through 2005 were infected perinatally. A number of factors are associated with an increased risk of an HIV-positive mother passing the infection to her baby. They include low CD4+ T cell count, high viral load (the concentration of virus in the blood), advanced HIV progression, presence of a particular HIV protein (p24) in serum, and placental membrane inflammation. Intrapartum (at the time of birth) events resulting in increased exposure of the baby to maternal blood, breastfeeding, low vitamin A levels, premature rupture of membranes, prenatal use of illicit drugs, and premature delivery also increase the risk of mother-to-child transmission. The risk of perinatal transmission also increases when the mother does not know she is infected until late in the course of the illness.

Despite these potential routes of transmission, the number of HIV-infected infants has been declining, probably as a result of the more widespread use of HAART to prevent pregnant women from passing HIV infection to their offspring. Planned cesarean section delivery (C-section; the surgical delivery of a baby), the presence of neutralizing antibodies in the mother, and timely antiviral drug therapy (such as with ZDV) further reduce the chances of mother-to-infant HIV transmission.

There is evidence that planned C-section, with the administration of ZDV, may prevent some cases of mother-to-child HIV infection. C-section delivery does not expose the baby to potentially HIV-contaminated vaginal tissue.

Even without a C-section, drug therapy can be beneficial. Edward M. Connor et al., in ''Reduction of Maternal-Infant Transmission of Human Immunodeficiency Virus Type 1 with Zidovudine Treatment'' (New England Journal of Medicine, vol. 331, no, 18, November 3, 1994), discuss the results of their study, which was the first study of perinatal transmission prevention and is considered a landmark study. Connor et al. conducted a randomized, double-blind, placebo-controlled (an inactive substance that does not contain drug; placebos are used in comparative studies to measure the effectiveness of an experimental drug or regimen) study of antiviral prophylaxis using ZDV. At eighteen months there was a dramatic risk reduction--two-thirds (67.5%)--in mother-to-child transmission. HIV transmission occurred in 25.5% of women who had received the placebo, compared to just 8.3% of those who received ZDV.

Even though HIV transmission from mother to child is the most common route of HIV infection in children and the source of almost all AIDS cases in children in the United States, the number of AIDS cases transmitted this way has decreased dramatically. The CDC notes in Mother-to-Child (Perinatal) HIV Transmission and Prevention (May 2006) that by 2004 there were only 145 reported cases of mother-to-child transmission in the United States. Figure 5.1 shows the decrease in mother-to-child transmission of AIDS from 2000 to 2004. According to the World Health Organization (WHO), in Taking Stock: HIV in Children (2006), in 2005 an estimated 540,000 children under fifteen years of age became infected with HIV, mostly through mother-to-child transmission. Infection usually occurs during the last stages of pregnancy, most often during labor and delivery.

James McIntyre states in ''Strategies to Prevent Mother-to-Child Transmission of HIV'' (Current Opinion in Infectious Diseases, vol. 19, no. 1, February 2006) that there is widespread agreement in the medical community that an effective approach to preventing mother-to-child transmission provides antiretroviral treatment in pregnancy and post-pregnancy to those women who need it, and an effective preventive regimen for those who do not yet need treatment. Preventive regimens include HAART, a ZDV-plus-nevirapine regimen in some settings, or nevirapine (a nonnucleoside reverse transcriptase inhibitor) alone in countries and settings where this is the only possible therapy.

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