Rh incompatibility occurs when a pregnant woman has Rh-negative blood and the fetus has Rh-positive blood.
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 see Sidebar 4: 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 Common causes). In severe cases, the brain may be damaged (called kernicterus—see Complications), 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.
At the first visit to a doctor during a pregnancy, women are screened to determine whether they have Rh-positive or Rh-negative blood. If they have Rh-negative blood, their blood is checked for Rh antibodies and the father's blood type is determined. If he has Rh-positive blood, Rh sensitization is a risk. In such cases, the woman's blood is checked for Rh antibodies periodically during the pregnancy. The pregnancy can proceed as usual as long as no antibodies are detected.
If antibodies are detected, steps may be taken to protect the fetus, depending on how high the antibody level is. Doppler ultrasonography may be done periodically to evaluate blood flow in the fetus's brain. If it is abnormal, the fetus may have anemia. Then doctors anesthetize an area of skin over the woman's abdomen and insert a needle through the abdomen into the umbilical cord to obtain a sample of blood from the fetus (a procedure called percutaneous umbilical blood sampling). The sample is then analyzed to check for anemia.
As a precaution, women who have Rh-negative blood are given an injection of Rh antibodies at 28 weeks of pregnancy and within 72 hours after delivery of a baby who has Rh-positive blood, even after a miscarriage or an abortion. They are also given an injection after any episode of vaginal bleeding and after amniocentesis or chorionic villus sampling. The antibodies given are called Rh0(D) immune globulin. This treatment works by making the woman's immune system less able to recognize the Rh factor on red blood cells from the baby, which may have entered the woman's bloodstream. Thus, the woman's immune system does not make antibodies to the Rh factor. Such treatment reduces the risk that the fetus's red blood cells will be destroyed in subsequent pregnancies from about 12 to 13% (without treatment) to 1 to 2%.
If anemia is diagnosed in the fetus, the fetus is given a blood transfusion before birth by a specialist at a center that specializes in high-risk pregnancies. Most often, the transfusion is given through a needle inserted into a vein in the umbilical cord. Usually, additional transfusions are given until the fetus is mature enough to be safely delivered.
Women are often given corticosteroids to help the fetus's lungs mature. Then the baby can be safely delivered earlier. When the fetal lungs are mature, labor is induced (usually at 32 to 35 weeks of pregnancy). The baby may need additional transfusions after birth. Sometimes no transfusions are needed until after birth.
Last full review/revision August 2013 by Antonette T. Dulay, MD