Rh incompatibility occurs when an Rh-negative mother is exposed to Rh-positive fetal blood (usually during the delivery of an Rh+ baby). This exposure causes the mother to produce antibodies against the Rh factor. In subsequent pregnancies with an Rh+ fetus, these maternal antibodies can cross the placenta and attack the fetal red blood cells, leading to hemolytic disease of the newborn (erythroblastosis fetalis).