What is mother-to-child transmission of HIV?

Mother-to-child transmission of HIV is the spread of HIV from a woman to her child during pregnancy or childbirth (also called labor and delivery) or in breast milk. Mother-to-child transmission is the most common way that children become infected with HIV. 

HIV medicines work in several ways to prevent mother-to-child transmission of HIV. HIV medicines reduce the amount of HIV in an HIV-infected pregnant woman’s body, which reduces the chances that her baby will be exposed to HIV during pregnancy and childbirth. HIV medicine also passes from a pregnant woman to her baby across the placenta (also called the afterbirth). This transfer of HIV medicine protects the baby from infection with HIV, especially near delivery when exposure to HIV in the mother's blood and other fluids is greatest. After birth, babies born to HIV-infected women receive HIV medicines to protect against infection from any HIV that passed from mother to child during childbirth. 

Source: aidsinfo