Researchers are not exactly sure when babies are infected with HIV during pregnancy. It has been said that a small percentage of all babies are infected with HIV while developing inside their mothers' uteruses (wombs). However, this has not really been proven. It is known that the vast majority of infections occur during labor (the time of delivery) or after the baby is born and is breast-fed by his or her HIV-infected mother.
Throughout pregnancy, a developing fetus has his or her own blood supply. In other words, the developing fetus does not come into contact with the blood of his or her mother. This helps protect the fetus from infections in the mother's blood, such as HIV. However, developing fetuses do receive nutrients and various proteins, such as immune system antibodies, from their mothers. While a mother's HIV may not enter the fetus, her antibodies to the virus will. These antibodies cannot harm the fetus, but will cause the baby to test "positive" to an HIV antibody test at birth.
At the time of birth (labor), a baby often comes into contact with his or her mother's blood. If the mother's blood enters the baby's body, this is when HIV can be transmitted.
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