ObjectiveTo evaluate the etiology and outcome of fetal hydrops of nonimmune origin diagnosed in utero during the first half of pregnancy.MethodsWe reviewed 45 cases of nonimmune fetal hydrops presenting between 11 and 17 weeks' gestation over a 4-year period.ResultsThe median gestational age at diagnosis of fetal hydrops was 14 weeks. Placental edema was most commonly associated with generalized skin edema. Ascites was also observed in four cases, but no case presented with pleural or pericardial effusion. The fetal karyotype was abnormal in 35 cases (77.8%). Of the ten fetuses with a normal karyotype, four were classified as idiopathic, three had isolated atrio-ventricular septal defect, two were associated with maternal infection, and one had multiple pterygium. Fetal heart rate anomalies were found in both chromosomally normal and abnormal fetuses. All but one of the karyotypically abnormal pregnancies and five of ten euploid pregnancies were terminated. In all six pregnancies that continued, resolution occurred before mid-gestation. Three continuing euploid pregnancies resulted in fetal death, and only two had a normal outcome.ConclusionNonimmune fetal hydrops diagnosed before 18 weeks' gestation is associated with a higher incidence of aneuploidy than hydrops diagnosed during the second half of pregnancy. In most affected fetuses with a normal karyotype, spontaneous resolution occurred before 24 weeks' gestation, although the outcome was generally unfavorable.