Vein of Galen aneurysmal malformation is a choroidal type of arteriovenous malformation that develops between 6 and 11 weeks of gestation. It results from 1 or more arteriovenous fistulas shunting blood toward the prosencephalic vein of Markowski, the embryonic precursor of the vein of Galen. This abnormal shunt leads to progressive dilation of the vein and prevents its involution and subsequent development into the vein of Galen. [from
HPO]