Rh Incompatibility

Rh incompatibility is a risk state, not a newborn diagnosis. It occurs when an Rh D-negative pregnant person carries an Rh D-positive fetus and fetal red blood cells enter the maternal circulation. The D antigen is present on Rh D-positive red blood cells but absent from Rh D-negative cells. A very small fetomaternal hemorrhage—approximately 0.1 mL or more of Rh D-positive fetal blood—may expose the maternal immune system to the D antigen. The person can then form anti-D antibodies, a process called alloimmunization or sensitization. The clinically dangerous antibody is usually anti-D IgG. IgG crosses the placenta. In a later exposure, it can attach to fetal Rh D-positive red blood cells and accelerate their destruction. The fetus may develop anemia; severe anemia can lead to heart failure, hydrops fetalis, and fetal loss. After birth, ongoing red-cell destruction produces rising unconjugated bilirubin and may cause significant jaundice. The first sensitizing exposure may not cause obvious disease, so “the first baby was unaffected” does not prove that a future pregnancy is safe. Sensitization can also occur through a previous pregnancy, transfusion with...