Rh incompatibility occurs when a pregnant woman has Rh-negative blood and the fetus has Rh-positive blood.
Rh incompatibility can result in destruction of the fetus’s red blood cells, sometimes causing anemia that can be severe.
The fetus is checked periodically for evidence of anemia.
If anemia is suspected, the fetus is given blood transfusions.
To prevent problems in the fetus, doctors give injections to women with Rh-negative blood late in the pregnancy, after delivery, and after certain procedures.
The fetus of a woman with Rh-negative blood may have Rh-positive blood if the father has Rh-positive blood. In about 13% of marriages in the United States, the man has Rh-positive blood and the woman has Rh-negative blood.
The Rh factor is a molecule on the surface of red blood cells in some people. Blood is Rh-positive if red blood cells have the Rh factor and Rh-negative if they do not. Problems can occur if the fetus’s Rh-positive blood enters the bloodstream of a woman with Rh-negative blood. The woman’s immune system may recognize the fetus’s red blood cells as foreign and produce antibodies, called Rh antibodies, to destroy Rh-positive blood cells. The production of these antibodies is called Rh sensitization.
During a first pregnancy, Rh sensitization is unlikely because no significant amount of the fetus’s blood is likely to enter the woman’s bloodstream until delivery. So the fetus or newborn rarely has problems. However, a woman becomes sensitized during delivery. Once she is sensitized, problems are more likely with each subsequent pregnancy if the fetus’s blood is Rh-positive. In each pregnancy, the woman produces Rh antibodies earlier and in larger amounts.
If Rh antibodies cross the placenta to the fetus, they may destroy some of the fetus’s red blood cells. If red blood cells are destroyed faster than the fetus can produce new ones, the fetus can develop anemia. Such destruction is called hemolytic disease of the fetus (erythroblastosis fetalis) or of the newborn (erythroblastosis neonatorum—see What Is Hemolytic Disease of the Newborn?). When red blood cells are destroyed, a yellow pigment called bilirubin is produced. When many red blood cells are destroyed, bilirubin can accumulate within the skin and other tissues. As a result, the newborn's skin and whites of the eyes may appear yellow (called jaundice—see Jaundice in the Newborn). In severe cases, the brain may be damaged (called kernicterus—see Jaundice in the Newborn), and severe anemia can result in the fetus's death. Miscarriage can occur.
Occasionally, other molecules on the woman's red blood cells are incompatible with those of the fetus. Such incompatibility can cause problems similar to those of Rh incompatibility.