Definition The Greek-Latin term hydrops fetalis is issued for pathologically increased fluid accumulations in fetal soft tissues and serous cavities.1 Immune hydrops fetalis refers to those cases of hydrops that are caused by alloimmune hemolytic anemia in the presence of circulating maternal antibodies against fetal erythrocytes.2 If there is no evidence of blood group incompatibility (isoimmunization), the hydrops is characterized as being nonimmune hydrops fetalis. The prenatal diagnosis of hydrops fetalis is achieved by ultrasound, which demonstrates the skin edema and/or fluid accumulations in serous cavities of the fetus (abdominal ascites, pleural and/or pericardial effusion). Fetal hydrops is defined by the demonstration of fluid accumulations in at least two of these four fluid compartments.3 But the placenta and the amniotic sac have sometimes been included as additional “fetal compartments.” Placentomegaly indicating hydrops placentae can be diagnosed if the placental thickness, from chorionic plate to base, increases by 4 cm or more in the second trimester and by 6 cm or more in the third trimester. Although polyhydramnios defined by an amniotic fluid index above 24 cm is associated in 30%–75% of cases with nonimmune hydrops,4,5 the amniotic sac should not be considered as one of the compartments for the diagnosis of hydrops, because the pathophysiological mechanisms of the occurrence of polyhydramnios can differ from those leading to an increased rate of interstitial fluid accumulation causing hydrops fetalis and placentae. Furthermore, hydrops may even be associated with oligohydramnios, for example, in some preterminal fetuses with Turner syndrome or in cases with intrauterine cytomegaloviral infection. Some series of nonimmune hydrops also include cases of isolated pleural effusion, abdominal ascites, or generalized skin edema, because fluid accumulation in one site may represent an early stage of a disease that may lead to fluid accumulation in several sites at a more advanced stage, especially in diseases known to result in generalized hydrops fetalis.