The variable expression of congenital CNS abnormalities makes their antenatal ultrasound diagnosis extremely difficult. Clinical decisions depend on ultrasound diagnosis; therefore, accurate antenatal diagnosis is imperative. A diagnostic algorithm based on ultrasonic demonstration of fetal cranial structures altered in congenital CNS abnormalities was constructed and applied retrospectively to 40 patients with clinicopathologic follow‐up adequate to define the cranial abnormality. Using the algorithm our diagnosis was correct in 37/40 (92 per cent) compared with 30/40 (75 per cent) without its use. The algorithm was highly accurate in the diagnosis of hydrocephalus (13/14), anencephaly/amniotic band syndrome (13/13), and holoprosencephaly (8/8). A wide spectrum of CNS abnormalities can be accurately diagnosed by ultrasonography in the antenatal period by the application of our algorithm.