Definition: Hydrops is defined by abnormal accumulation of serous fluid in skin (edema) and body cavities (pericardial, pleural, or ascitic effusions). Prevalence: Hydrops fetalis is found in about 1 per 2,000 births. Etiology: Hydrops is a non‑specific finding in a wide variety of fetal and maternal disorders, including hematological, chromosomal, cardiovascular, renal, pulmonary, gastrointestinal, hepatic and metabolic abnormalities, congenital infection, neoplasms and malformations of the placenta or umbilical cord. Hydrops is classically devided into immune (due to maternal hemolytic antibodies) and non-immune (due to all other etiologies). With the widespread introduction of immunoprophylaxis and the successfull treatment of Rhesus disease by fetal blood transfusions, non‑immune causes have become responsible for at least 75% of the cases, and make a greater contribution to perinatal mortality. While in many instances the underlying cause may be determined by maternal antibody and infection screening, fetal ultrasound scanning, including echocardiography and Doppler studies, and fetal blood sampling, quite often the abnormality remains unexplained even after expert post‑mortem examination. Prognosis: Although isolated ascites, both in fetuses and neonates, may be transitory, the spontaneous resolution of hydrops has not been reported and the overall mortality for this condition is about 80%.