Hemolytic disease of the newborn: Abnormal breakup of red blood cells in the fetus or newborn. This is usually due to antibodies made by the mother directed against the baby's red cells. It is typically caused by Rh incompatibility, that is differences between the Rh blood group of the mother and baby. Severe hemolytic disease can cause anemia and heart failure. Less-severe cases include jaundice, and can lead to brain damage is left untreated. Also known as erythroblastosis.