In underdeveloped countries, about half of HIV-infected people are women. This leads to mother-to-child (vertical) transmission of infection. The number of children born to HIV-positive mothers is more than a seventeen hundred a day and more than seven million children under fifteen have been infected by the virus. In many areas of sub-Saharan Africa, more than a third of women are infected. One third to one half of mother to child transmissions of HIV results from breast feeding and about a quarter of babies born to HIV-infected mothers are themselves infected. Greater than 90% of mother-to-child transmission takes place in developing countries since in western countries chemotherapy has made vertical transmission very rare (less than 2%). 

How do babies acquire the virus? This can occur before or after birth. Before birth, the virus can be acquired across the placenta by hemorrhage or from the amniotic fluid. At birth, the baby can come in contact with maternal blood or vaginal secretions and finally, the virus can contaminate the mother’s milk. The rate of vertical transmission depends on a number of factors but primarily it depends upon the virus load of the mother. This is highest if the mother is in advanced state of disease (with concomitant low levels of maternal antibodies) or in the high viral load stage that precedes seroconversion. If she is malnourished, has other sexually transmitted diseases or if the baby is in contact with vaginal fluids for a long time during labor, the rate of transmission rises. Prematurity is also a risk factor for transmission, perhaps because of the lack of development of the baby’s immune system. The longer the breast feeding period is maintained the greater is the risk of vertical transmission. 

There are a variety of ways that vertical transmission of HIV can be reduced, and as with other forms of HIV transmission, education is very important. However, anti-retroviral prophylaxis is paramount in lowering the rate of transmission. Chemotherapy is given both antenatally to the mother and postnatally to the infant. 

AZT was the first drug to be shown to reduce the rate of transmission. The rate of transmission fell from 22% to under 8% in one study, when the drug was given intravenously to the mother and then via formula feeding to the infant. A regimen that includes oral AZT started at 14-34 weeks of gestation and continued until delivery, followed by intravenous AZT during labor and six weeks of oral AZT to the baby resulted in a 70% reduction in the risk of vertical transmission. A combination of AZT and 3TC with such a regimen showed a transmission rate of just 1.6% which is similar to that in western countries. However, such a regimen is often impractical in developing regions. 

The use of nevirapine substantially reduces transmission in even single dose therapy; in this the mother receives 200mg nevirapine at the time of labor and a dose of 2mg/kg is given to the baby at 72 hours. This is safe and efficacious and reduces transmission by 50% to around 10-15%. The drug is available free from the manufacturer in resource-poor areas. In an extension of this, a short course of AZT plus 3TC, together with single dose nevirapine has shown 6-week transmission rates below 5% in a population in which 40% breast-feed. Unfortunately, more than 75% of women receiving nevirapine develop a major nevirapine resistance mutation that prohibits subsequent use of this drug in the treatment of the mother. 

Breast feeding is a major problem in post-natal transmission of HIV to the infant and half of infections may result in his way. The probability of infection is around 10%. This can, of course, be avoided by cessation of breast feeding but is often impractical in third world countries. In a trial in Uganda, in which there was a limited regimen of ante-natal drug treatment, exclusively breast fed infants showed a transmission rate of 16% while those exclusively formula fed had a rate of 3.7%.