Definition of Rh incompatibility:

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.

Causes, incidence, and risk factors:

During pregnancy, red blood cells from the unborn baby can cross into the mother's bloodstream through the placenta.

If the mother is Rh-negative, her immune system treats 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 back through the placenta into the developing baby and destroy the baby's circulating red blood cells.

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

Because it takes time for the mother to develop antibodies, firstborn infants are often not affected unless the mother had past miscarriages or abortions that sensitized her immune system. However, all children she has afterwards who are also Rh-positive may be affected.

Rh incompatibility develops only when the mother is Rh-negative and the infant is Rh-positive. Thanks to the use of special immune globulins called RhoGHAM, this problem has become uncommon in the United States and other places that provide access to good prenatal care.