In HIV-positive pregnant women, the virus is found abundantly in the birth canal (cervix and vagina) and in the mother's blood. Therefore, if the baby is exposed to vaginal fluid or to the mother's blood during labour and delivery, there is an increased chance of MTCT occurring.

Normally, there is no direct mixing between the maternal and fetal blood in the uterus, as you learnt in Study Session 5. However, anything that breaks the barrier between the placenta and the wall of the uterus will increase the risk of MTCT of HIV. Box 16.1 lists some of the factors during pregnancy that can damage the barrier between maternal and fetal blood supply in the placenta, increasing the risk of MTCT. You will learn more about risk factors that increase the chance of MTCT of HIV during labour and delivery, and during lactation, in the next two Modules on Labour and Delivery Care and Postnatal Care.

Box 16.1 Damage to the barrier between fetal and maternal blood supply in the placenta

Common factors that damage the natural barrier between the fetal and maternal blood supply in the placenta and expose the fetus to maternal blood include:

Infection of the placenta due to malaria, or by bacteria or viruses.
Bleeding from the placenta before labour begins (the medical name for this is antepartum haemorrhage). This can occur due to placental abruption (placenta detaching too early from the uterus) or placenta previa (placenta covering the opening of the cervix). You will learn about these placental conditions in Study Session 21.
Injury to the abdomen due to a blow, or by a sharp object which penetrates the abdomen.
Vigorous abdominal massage by traditional healers in late third trimester. In some areas of Ethiopia, traditional healers repeatedly massage the abdomen, which they believe will make delivery of the baby easier.
Maternal malnutrition, especially deficiency of vitamin C, vitamin A, or the mineral zinc.
Cigarette smoking, which weakens the fetal membranes surrounding the unborn baby and increases the chance of developing placental abruption.

One question you may have is whether pregnancy affects HIV disease progression, or whether HIV disease progression affects pregnancy. So far, the evidence suggests that pregnancy does not make HIV worse. Similarly, there is no evidence that HIV infection leads to bad pregnancy outcomes.