Erythroblastosis fetalis, also known as Rh Disease or Hemolytic Disease of the Newborn, occurs when there is blood incompatibilities between the pregnant woman and her unborn baby. While it occurs in ABO incompatibility, it most often has to do with the Rhesus (Rh) factor in the blood. If a mother is Rh-negative and the fetus is Rh-positive, the mother's body will make antibodies that will work to attack the fetus's red blood cells. Also, the fetus will make an excessive amount of red blood cells to make up for the loss of the red blood cells that the mother