In a prospective study of 162 patients, measurements of the transverse cerebellar diameter (TCD) and abdominal circumference (AC) were obtained between 15-38 weeks' gestation. A ratio between the TCD and AC was calculated after each examination. This ratio remained constant during gestation. The mean ratio was 13.7% (fifth and 95th percentiles of 11.9 and 15.9%, respectively); the median ratio was 13.6%. Forty-two patients had repeat measurements and the ratio did not change. This ratio is gestational age-independent; however, further investigation is needed to determine its application in the assessment of fetal growth abnormalities.