Erythroblastosis fetalis is a severe anemia that develops in an unborn infant because the mother produces antibodies that attack the fetus' red blood cells. The antibodies are usually caused by Rh incompatibility between the mother’s blood type and that of the fetus (that is, the mother and baby have different blood types).

The severity of this condition can vary widely. In some instances this can lead to death of the baby. It can be treated in utero (before birth) by medication or intrauterine transfusion. When the child is born, signs may include an enlarged liver and/or spleen, generalized edema (swelling, anasarca), jaundice, and anemia. After birth, depending on the severity, a transfusion may be performed.

See also Transfusion reaction.