HIV infection can be transmitted from an infected mother to her fetus during pregnancy, delivery, or by breast-feeding. Where the proportion of infected women to infected men is almost 1:1, it is an extremely important form of transmission of HIV infection in many developing countries. HIV can be transmitted to the fetus as early as the first and second trimester of pregnancy, proved by virologic analysis of aborted fetuses of infected mothers. Transmission to the fetus occurs most commonly in the perinatal period (a month before and a month after birth). Generally mother-to-child transmissions are 23–30% before birth, 50–65% during birth, and 12–20% via breast-feeding.
The probability of transmission of HIV from mother to infant/fetus ranges from 15–25% in developed countries and from 25–35% in developing countries due to absence of prophylactic antiretroviral therapy to the mother during pregnancy, labor, and delivery. These differences may be due to adequacy of prenatal care, general health of the mother during pregnancy and the stage of HIV disease. Higher rates of transmission have been reported with the presence of high maternal levels of plasma viremia (presence of virus).
Breast feeding is an important mode of transmission of HIV infection in developing countries, where mothers continue to breast-feed for long periods. Factors that increase the chance of transmission include detectable levels of HIV in breast milk, the presence of mastitis (infection of breast), low maternal CD4+T cell counts, and maternal vitamin A deficiency. The risk of HIV infection via breast-feeding is highest in the early months of breast-feeding. Exclusive breast feeding has been reported to carry a lower risk of HIV transmission than mixed feeding. The mechanism is not clearly understood. In developed countries, breast-feeding by an infected mother should be avoided. But, breast milk is the only source of adequate nutrition as well as immunity against potentially serious infections for the infant in developing countries. Present trend is to continue breast feeding, along with anti retroviral (anti HIV) therapy.
A prolonged interval between membrane rupture and delivery is well-documented risk factor for transmission. Other conditions that are potential risk factors include STDs during pregnancy; hard drug use during pregnancy; cigarette smoking; preterm delivery; and obstetrical procedures such as amniocentesis, amnioscopy, fetal scalp electrodes, and episiotomy. Zidovudine treatment of HIV infected pregnant women; from the beginning of the second trimester through delivery and for 6 weeks following delivery reduce the risk of transmission. Anti HIV treatment combined with cesarean section delivery, has reduced HIV transmission dramatically in developed nations. Universal voluntary HIV testing and counseling of all pregnant women should be done. Zidovudine alone or in combination with lamivudine given to the mother during the last few weeks of pregnancy or even only during labor and delivery, and to the infant for a week reduced transmission to the infant. In developing countries a single dose of nevirapine given to the mother at the onset of labor and a single dose to the infant within 72 h of birth, is recommended to prevent transmission. This strategy has reduced HIV transmission to the fetus, and protecting against subsequent transmission by breast-feeding.
In the United States, the rate of mother-to-baby transmission was 0% among women with less than 1000 copies of HIV RNA per milliliter of blood, 16.6% among women with 1000–10,000 copies of HIV RNA per milliliter; 21.3% among women with 10,001–50,000 copies of HIV RNA per milliliter; 30.9% among women with 50,001–100,000 copies of HIV RNA per milliliter; and 40.6% among women with >100,000 copies of HIV RNA per milliliter. But there is no lower “threshold” below which transmission never occurs, because some studies have reported transmission at as low as less than 50 copies of HIV RNA per milliliter. Low maternal CD4+ T cell count is also attributed to increase HIV transmission, because low count is associated with high viremia. Closer HLA match between mother and child is associated with increased mother-to-child transmission.