My biology teacher says that a baby doesn't share blood with its mother while it's still in the womb. But I think that can't be true, because I know that a pregnant woman with AIDS can pass it to her baby, and AIDS is carried in blood. Can you explain this to me?

A.L.

Your biology teacher is correct, for the most part. Blood itself is not supposed to cross the placenta (the organ that develops in the uterus during pregnancy and nourishes the growing fetus) and pass from mother to fetus. Certain proteins carried in the blood clearly do cross the placenta from the mother, notably maternal antibodies (disease-fighting proteins) that provide some immunity to the growing fetus. In addition, many medications can pass from mother to fetus. This is often unwanted, because some drugs can cause birth defects or other problems in the baby. Sometimes, however, we depend on medications to cross the placenta to protect the unborn baby. When we prescribe a drug to treat toxoplasmosis in the pregnant woman, for example, we expect that the fetus may be treated as well.

While the fetus is not typically exposed to maternal blood, exposure can occur under certain circumstances. Sometimes an accident in the womb results in damage to the placenta, allowing blood to pass from mother to fetus. In these circumstances, fetal blood can also pass to the mother. When this occurs, antibodies produced by the mother's immune system may recognize fetal blood as foreign to the mother, causing anemia and other significant reactions in the baby. These small placental injuries are thought to make up only a small number of instances of where HIV (the virus that causes AIDS) is transmitted from mother to baby. Some infections, such as toxoplasmosis, can infect the fetus without any blood passing between the mother and fetus. The organism infects the placenta, and from there it passes to the fetus.

Most instances of blood transference from mother to fetus occur at the time of delivery. As most people know, delivery is a bloody process. Several milliliters of blood are often passed between mother and baby. This is where most cases of HIV and hepatitis B are transmitted. Simply giving the mother the drug AZT (zidovudine, trade name Retrovir) during labor can largely prevent HIV transmission from mother to fetus. AZT is even more effective when given to the mother throughout pregnancy and to the baby after birth. Some experts believe that delivering a baby by cesarean section may decrease the risk of blood exposure and thus lower the risk of HIV even further.