We report the case of a 27-year-old pregnant woman in whom isolated mild fetal cardiomegaly, diagnosed prenatally on sonographic examination at 22 weeks' menstrual age, was the first sign of development of an arteriovenous malformation of the vein of Galen. The arteriovenous malformation was visualized on sonographic examination at 29 weeks' menstrual age; prenatal MRI at 32 weeks confirmed the diagnosis. At 34 weeks' menstrual age, repeat sonographic examination demonstrated polyhydramnios, cardiomegaly, and generalized hydrops with ascites and pleural effusion. After vaginal delivery, the male neonate died of respiratory distress. We present the findings of prenatal gray-scale, color Doppler, and power Doppler sonography; MRI; and postmortem examination.