Our definition of hydramnios is an amniotic fluid index of 24 cm or greater. We evaluated by ultrasound examination 49 consecutive patients who met this definition of hydramnios. Of these, 22 (44.9%) had anomalies visible by ultrasound. The combination of hydramnios, abnormal hand posturing, and any other anomaly created a constellation of sonographic findings enabling us to predict six specific autosomal trisomies (27.27%): three trisomy 18, two trisomy 21, and one trisomy 13. The 27 fetuses with "idiopathic hydramnios" (no identifiable anomaly or abnormal hand posturing) had normal karyotypes. We recommend that any patient with confirmed hydramnios have a detailed ultrasound examination by an experienced sonographer, with special attention paid to the heart, face, and hands. If no abnormality is seen and the hands are normally postured, expectant management may be appropriate. If the late second- or third-trimester fetus displays abnormal hand posturing with any other abnormality, rapid karyotyping by funipuncture or placental biopsy should be recommended to facilitate appropriate management.