Hydrops fetalis: Gross edema (swelling), usually with anemia, of the fetus. It can be due to Rh blood group incompatibility, in which antibodies crossing the placenta from the mother destroy the red blood cells of the fetus. It can also be caused by a lethal form of the genetic disorder thalassemia, alpha thalassemia, in which alpha-chain polypeptides needed to make fetal or adult hemoglobin are not produced. See also Rh incompatibility; thalassemia, alpha.
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