Hydrops fetalis (fetal hydrops) is a serious fetal condition defined as abnormal accumulation of fluid in two or more fetal compartments, including ascites, pleural effusion, pericardial effusion, and skin edema. In some patients, it may also be associated with polyhydramnios and placental edema. Hydrops is usually first recognized by ultrasonographic examination during the first or second trimester of gestation. Significant fluid collections are easily detected, but fluid accumulation may also be limited and thus escape routine ultrasonographic detection.
Hydrops fetalis has been a well-recognized fetal and neonatal condition throughout history. Until the latter half of the 20th century, it was believed to be due to Rhesus (Rh) blood group isoimmunization of the fetus. More recent recognition of factors other than isoimmune hemolytic disease that can cause or be associated with fetal hydrops led to the use of the term nonimmune hydrops to identify those cases in which the fetal disorder was caused by factors other than isoimmunization.
In the 1970s, the major cause of immune hydrops (ie, RhD antigen) was conquered with the use of immunoglobulin (Ig) prophylaxis in at-risk mothers. Before routine immunization of Rh-negative mothers, most cases of hydrops were due to erythroblastosis from Rh alloimmunization. With the introduction of widespread immunoprophylaxis for red blood cell alloimmunization and the use of in-utero transfusions for immune hydrops therapy, nonimmune causes have become responsible for at least 85% of all cases of fetal hydrops. Nevertheless, in developing countries, the incidence of immune fetal hydrops is still high.