How is HIV transmitted from mother to child?

Without intervention, between 25% and 35% of the children born to HIV-positive mothers will themselves be infected.

In about 50% of the cases, transmission from mother to child occurs during labor and delivery. Scientists don't yet understand how exactly that transmission happens, but they have found that some treatments can prevent most cases. However, to many HIV-positive pregnant women in developing countries, these treatments are not available or acceptable. The hope is that better understanding of mother-to-child transmission will lead to more effective, more affordable, and more acceptable treatments. A study by Jesse Kwiek and colleagues (of the University of North Carolina) published in the international open-access medical journal PLoS Medicine implicates placental microtransfusions in HIV transmission.

Placental microtransfusions cause exchanges of small amounts of blood between the mother and the baby. They occur in most pregnancies once labor starts, because the contractions cause small areas of rupture in the placenta. However, the overall amount of blood exchanged differs from delivery to delivery. Until recently, it was not possible to measure the extent of these microtransfusions for a particular delivery, but now scientists have developed an assay based on umbilical cord blood that can do this. In this study, the researchers made use of this new assay to ask whether there is a link between the extent of placental microtransfusions and the likelihood of HIV transmission.

Kwiek and colleagues studied a group of mothers and children in Malawi. All of the mothers were HIV-positive, and some of them transmitted the virus to their children. This transmission occurred either during the pregnancy or around delivery, and the researchers knew the timing for each case. They also knew how the children were born: approximately three quarters by vaginal delivery and one fifth by emergency caesarean section. The researchers determined the level of placental microtransfusions from the umbilical cord blood and then looked for correlations between HIV transmission and the level of microtransfusions. They found no correlation for the cases where HIV was transmitted during pregnancy. For cases of transmission around delivery, however, higher levels of microtransfusions were associated with a higher risk of HIV transmission for vaginal deliveries.

If a connection between microtransfusions and transmission is confirmed by other studies, it might help to improve the timing of short-term prophylaxis regimens and possibly lead to the development of new strategies for preventing mother-to-child transmission of HIV.