Hydrops foetalis or hydrops fetalis is a condition in the fetus characterized by an accumulation of fluid, or edema, in at least two fetal compartments. By comparison, hydrops allantois or hydrops amnion is an accumulation of excessive fluid in the allantoic or amniotic space, respectively. Signs and symptoms Locations can include the subcutaneous tissue on the scalp, the pleura (pleural effusion), the pericardium (pericardial effusion) and the abdomen (ascites). Edema is usually seen in the fetal subcutaneous tissue, sometimes leading to spontaneous abortion. It is a prenatal form of heart failure, in which the heart is unable to satisfy demand (in most cases abnormally high) for blood flow. Causes Hydrops fetalis usually stems from fetal anemia, when the heart needs to pump a much greater volume of blood to deliver the same amount of oxygen. This anemia can have either an immune or non-immune cause. Non-immune hydrops can also be unrelated to anemia, for example if a fetal tumor or congenital cystic adenomatoid malformation increases the demand for blood flow. The increased demand for cardiac output leads to heart failure, and corresponding edema. Immune pathophysiology Erythroblastosis fetalis, also known as Rh disease, is the only immune cause of hydrops fetalis. Rh disease is a hemolytic disease of newborns. Pregnant mothers do not always have the same blood type as their child. During birth or throughout the pregnancy, the mother may be exposed to the infant's blood. In the event of a pregnancy where the fetus has the Rh-D blood antigen and the mother does not, the mother's immune system will respond to the red blood cells as foreign and create antibodies against the Rh-D antigen on the fetal blood cells. Rh disease develops in the event of a second pregnancy where the mother's immune system launches an attack, via IgG, against the infant's Rh-D positive blood cells. The immune response results in hemolysis of fetal red blood cells causing severe anemia. Hemolysis caused by the Rh incompatibility, causes extramedullary hematopoiesis in the fetal liver and bone marrow. The push to make more erythroblasts to help compensate with the hemolysis over works the liver causing hepatomegaly. The resulting liver dysfunction decreases albumin output which in turn decreases oncotic pressure. Consequentially, the decrease in pressure results in overall peripheral edema and ascites. Rh disease is currently an uncommon cause of immune-mediated hydrops fetalis. Due to preventative methods developed in the 1970s, the