Asymmetry and anomalies of the circle of Willis in fetal brain. Microsurgical study and functional remarks.

The arteries of the circle of Willis in fetal brains of 20-40 weeks of gestational age were studied with the operating microscope. The majority of fetuses were between 24-32 weeks of age. Our findings were as follows: 1) the average diameters of all segments were 4-5 times narrower than the average diameters in adults, and the average lengths of the same segments were 2-2.5 times shorter than the same lengths in adults; 2) the number of perforators corresponded with the perforating arteries in adults; 3) hypoplastic segments, as well as anomalies and variations, were identified; 4) calibers almost equal in size between the right and left corresponding segments were found in a high proportion of cases; 5) in the majority of cases a polygon, much like the "precarious" circle in adults, was dissected. The "fetal" or the "ideal" circles were found in a low proportion of cases.