Aneurysm of the vein of Galen (AVG), a complex malformation that has a generally poor prognosis, with a very high risk of neurologic sequelae and a mortality rate of 50%, represents less than 1% of cerebral arteriovenous malformations. The most noticeable prenatal finding is the existence of a posterior, midline, or slightly lateral supratentorial cystic structure in the fetal brain. Visualization of the blood flow in the cystic structure facilitates the diagnosis of AVG; prenatal magnetic resonance imaging is useful for evaluating the vascular connection and the degree of cerebral damage. In addition, prenatal findings indicative of heart failure are associated with a poor prognosis for the newborn. Therefore, early detection is important in order to proceed to effective therapeutic management. Premature delivery does not improve the perinatal outcome. The prenatal diagnosis of AVG has been reported in only a few occasions. In this report, we present a prenatal diagnosis of AVG with cardiomegaly in a fetus at the 31 weeks’ gestation.