Rh disease is one of the causes of hemolytic disease of the newborn (also known as HDN). The disease ranges from mild to severe. When the disease is mild the fetus may have mild anaemia with reticulocytosis. When the disease is moderate or severe the fetus can have a more marked anaemia and erythroblastosis (erythroblastosis fetalis). When the disease is very severe it can cause morbus haemolyticus neonatorum, hydrops fetalis, or stillbirth.