Hydrops fetalis describes edema in a developing fetus in the uterus that appears in at least two different locations. It can occur below the skin, in the space around the lungs, in the pericardium or the abdomen. The condition often leads to miscarriage and results from a partial failure of the fetal heart which then sets off the edema. It is often the result of anemia in the developing fetus, forcing the fetal heart to pump faster than it can and starving the developing tissues of blood. It can also result when a tumor or other growth that increases the demand for blood.
There are several causes for hydrops fetalis, which are categorized into "immune" and "non-immune". The only "immune" cause is a mis-match between the blood type of the fetus and the mother, in particular the Rh factor. There are several non-immune causes, including iron deficiency, certain enzyme deficiencies, parvovirus, syphillis, Turner Syndrome, teratoma, and hyperthyroidism.