How does an HIV positive lady not pass HIV to her unborn child?

Im not HIV positive myself but am very curious as to how there is such a high chance that an HIV positive mother will not pass HIV to her unborn baby? Isnt the baby being fed threw her blood supply? If the mothers blood is flowing into the baby then how is it the baby does not contact the diesese?

The placenta does a pretty good job of filtering the mother's blood before allowing it to enter the fetus' blood. It allows antibodies to pass through, but keeps out most other things. The placenta does a good job of keeping HIV out. However, anything that affect the integrity of the placenta can increase risk, especially issues involving inflammatory processes (eg. premature rupture of fetal membranes, spontaneous preterm delivery and the presence of chorioamnionitis.)

Most fetus' are infected during the birthing process when they come into contact with unfiltered HIV+ blood......however, if the woman is on HIV meds and her viral load is undetectable (read: very low) then the chance that she will pass HIV onto her baby is less than 1%. In fact, with an undetectable viral load and HIV meds, a C-section is not (according to recent research) required in most cases . The mother's viral load is the most important risk factor for mother to child transmission.

ps. I should let you know that the placental barrier in relation to HIV is still actively being studied and new research may give us more insite into the exact mechanisms of mother to child transmission

If the HIV status of the mother is known, then she will be placed on antiretroviral drugs (AZT is the most common) as a measure to help in lowering the risk of transmission. Transmission in the body is also dependent on such factors such as the viral load (or amount of HIV in the blood) and the CD4 cell counts (cells that play a role in the immune system) A high viral load and low CD4 count increases the risk of transmission.

Factors during the pregnancy can either hinder or help reduce the risk of transmission. These include the mode of birth (C-section or vaginal), duration of labor, and the exposure to cervical and vaginal secretions (can contain virus).

By having a C-section. This way the baby does not pass through the birth canal. Remember the baby is protected in its own fluid which is within the mother's body. That way, when doctors perform C-sections the baby does nit have to be infected