Erythroblastosis fetalis is a hemolytic disease of the newborn caused by Rh incompatibility, where an Rh-negative mother carries an Rh-positive fetus.
Named for the erythroblasts (nucleated red cells) released into the blood from the hemolysis.
Etiopathogenesis
Step 1 An Rh-negative mother carries an Rh-positive fetus; no reaction occurs in the first pregnancy.
Step 2 At delivery, placental separation lets a small amount of fetal blood leak into the maternal circulation, inducing maternal anti-Rh agglutinin formation.
Step 3 In subsequent pregnancies, the maternal anti-Rh agglutinin, predominantly IgG, crosses the placenta into the fetal circulation and causes hemolysis.
Step 4 Hemolysis worsens with each subsequent pregnancy, most severe by the third pregnancy onward.