Fetal edema (abnormal fluid buildup), also known as hydrops fetalis, is a serious condition that causes body swelling in a fetus or newborn. There are two types of hydrops fetalis: immune and nonimmune. Immune hydrops fetalis is caused by a severe Rh incompatibility between mother and fetus. The mother has an Rh negative blood type that makes antibodies that attack the fetus’ Rh positive blood cells and destroys them leading to severe swelling that affects the fetal organs. Nonimmune hydrops fetalis is the more common type (90%), which occurs when the fetus or newborn has a disease or medical condition, such as heart or lung disorders, severe anemia, blood conditions or infections, or genetic or developmental problems. Hydrops fetalis can be mild or severe.