Rh Disease
From Encyclopedia of Sex and Sexuality
A complication of pregnancy in which the red blood cell types of the pregnant woman are incompatible with the fetus she is carrying. On their surface red blood cells are covered by antigens, structural markers that give the cells a distinct immunological identity. All the red blood cells in a person’s body have the same surface antigens, but these surface markers differ from person to person. When a woman is exposed to blood that is unlike her own (most commonly as a result of the passage of fetal red blood cells across the placenta or from a blood transfusion), she develops antibodies to destroy the foreign blood in her circulation as part of a normal immune response.
Called Rh disease because it was first studied in rhesus monkeys, this incompatibility most commonly occurs when a woman lacks the D antigen in her red blood cells (“Rh negative”) and carries a fetus with the D antigen (“Rh positive”). However, there are dozens of other, less common blood cell antigens that can cause the same clinical problems.
The antibodies produced persist for life and, during pregnancy, can cross the placenta and destroy fetal red blood cells if they are positive for the antigen that caused the original immune reaction. This results in fetal anemia, ranging from only mild to severe anemia, in which hydrops fetalis (an accumulation of fluids throughout the fetal body) or even fetal death occurs.
Woman who are Rh sensitized are usually identified by routine testing for red blood cell antibodies during early pregnancy. If it is present, the amount of antibody is determined; if it is below a certain level (usually 1 to 8), then it is not likely that the fetus is severely anemic. If the level of antibody is above this threshold, the severity of fetal anemia must be more closely evaluated. Usually, the extent of the anemia is indirectly estimated by analyzing amniotic fluid obtained by amniocentesis. In some cases, the fetal red blood cell count is measured directly by drawing blood from the umbilical cord in a procedure like amniocentesis known as umbilical cord blood sampling.
If testing indicates that the fetus is severely anemic and if the pregnancy is not close enough to term to permit a safe delivery, a transfusion is performed. Transfusion is done by injecting Rh negative red blood cells into the fetal abdomen or directly into the fetus’s blood stream by way of the umbilical cord. The procedure is repeated at regular intervals until delivery.
Fortunately, Rh disease can largely be prevented by giving Rh negative pregnant women an injection of Rh immune globulin at about twenty-eight weeks of gestation and at any time during pregnancy or after delivery (if their baby is Rh positive). No such protective treatment is available for the minor antigens.
