• Pre-requisite of Rh isoimmunization: Rh- mother and Rh+ fetus.
• The mother is sensitized to Rh antigen by Rh+ fetal erythrocytes that reach the maternal circulation. The mother is thus stimulated to production of anti-Rh antibodies. Maternal IgG antibodies cross the placenta, attach to the red blood cells and lead to hemolytic anemia in the fetus. Compensatory mechanisms used by the fetus include extensive activation of the hematopoeitic tissue and increased cardiac output. Congestive heart failure and generalized edema are marks of decompensation. The sensitization occurs most often during the first childbirth, so the second and any following pregnancy are at risk of hemolytic disease. The immunization also develops following invasive examination durign pregnancy and abortion but the volume of transplacental bleed is lower than during childbirth. There is a very effective preventive program of Rh-isoimmunization. Intravenous drug abuse in young women has been reported as the most common cause of Rh isoimmunization today.

Rare causes of erythroblastosis fetalis: Isoimmunisation against other antigens of the Rh system (C, E) or antigens Kell, Lewis occurs rarely. The clinical signs are the same as for the original anti-D disease.

ABO incompatibility: Pre-requisite: Mother of group O and infant of group A or B. Hemolytic disease of the newborn in cases of ABO incompatiblity is usually mild. It presents as fetal anemia in the third trimestr of pregnancy and moderate jaundice after delivery. Hydrops fetalis is an exceptional complication.