Hydrops fetalis is the term applied to the presence of excess body water in the fetus resulting in skin edema or effusions in the pleural, peritoneal, or pericardial space. Because surveillance during most pregnancies in the USA includes fetal ultrasound, most cases of hydrops will be recognized before birth. An associated abnormality can be diagnosed either antenatally or postnatally in the majority of patients who have hydrops. The prognosis for survival is generally poor. Over 50% of fetuses diagnosed with hydrops die in utero, and of those that survive to delivery, over half will die postnatally despite aggressive support.

Immune hydrops

Immune hydrops is a late manifestation of the destruction of fetal erythrocytes and resultant anemia caused by transplacentally acquired maternal antibodies to fetal red cell antigens. The degree of anemia that causes hydrops is unpredictable, but hydrops most commonly occurs when the hematocrit is less than 20%. Immune hydrops not treated with intrauterine red cell transfusion is associated with a significant risk of fetal death.

Historically, the most common antigen causing an antibody-mediated hemolytic anemia was the Rh D. Anemia as a function of sensitization to the D antigen is infrequent today because of the routine use of passive immunization with Rh immunoglobulin in the management of women who are Rh D-negative. Sensitization to other red cell antigens, including Kell, e and c, also cause fetal hemolytic anemia and immune hydrops.