Rh incompatibility is a condition that develops when a pregnant woman has Rh-negative blood and the baby in her womb has Rh-positive blood.

During pregnancy, red blood cells from the fetus can get into the mother's bloodstream as she nourishes her child through the placenta.

If the mother is Rh-negative, her immune system treats the Rh-positive fetal cells as if they were a foreign substance and makes antibodies against the fetal blood cells. These anti-Rh antibodies may cross the placenta into the developing baby, where they destroy the baby's circulating red blood cells.

When red blood cells are broken down, they make bilirubin, which causes an infant to become yellow (jaundiced). The level of bilirubin in the infant's bloodstream may range from mild to dangerously high.

First-born infants are often not affected -- unless the mother has had previous miscarriages or abortions, which could have sensitized her system -- as it takes time for the mother to develop antibodies against the fetal blood. However, second children who are also Rh-positive may be harmed.

Rh incompatibility develops only when the mother is Rh-negative and the infant is Rh-positive. This problem has become uncommon in the U.S. and other places that provide good prenatal care. Special immune globulins, called RhoGAM, are now used to prevent RH incompatibility.