Real-time sonography: ventricular and vascular anatomy of the fetal brain in utero.

Detailed fetal intracranial anatomy can be examined in utero using real-time sonography. The ability to demonstrate motion makes it possible to visualize important vascular landmarks which are helpful in identifying adjacent structures and in recognizing appropriate planes of section. The lateral ventricular system can be demonstrated and the lateral ventricular ratio can be determined in cases beyond 27 weeks gestation if the head is in an occiput transverse position. A lateral ventricular ratio greater than 0.45 (bodies) or greater than 0.65 (atrium) should raise the question of hydrocephalus and close follow-up measurements of the biparietal diameter, the head circumference/abdominal circumference ratio, and the lateral ventricular system should be made to evaluate this possibility. The fetal third ventricle is not routinely visualized before 34 weeks gestation in the absence of hydrocephalus; alternative anatomic explantations for the structure commonly referred to in the literature as the fetal third ventricle are discussed.