The transverse diameter of the cerebellum was measured with ultrasound in 265 normal fetuses ranging from 15 to 39 weeks gestational age and found to correlate closely with the biparietal diameter. The transverse diameter may be useful in estimating fetal age, particularly in breech presentation where extrinsic pressure may deform the skull and decrease the biparietal diameter. Failure to demonstrate the cerebellum could be a sign of Arnold-Chiari malformation or Dandy-Walker cyst.