ObjectiveTo determine whether cerebellar hypoplasia in Down syndrome is established and clinically recognizable in the second trimester of pregnancy and to evaluate the screening utility of transverse cerebellar diameter measurements for Down syndrome fetuses.MethodsUltrasonographic biometry data obtained before genetic amniocenteses on 42 fetuses with Down syndrome and 1161 karyotypically normal fetuses were analyzed. Mean transverse cerebellar diameters stratified by gestational age were compared. A regression equation relating transverse cerebellar diameters to gestational age was calculated for 387 normal fetuses and applied to the remaining normal (n= 774) and all Down syndrome fetuses. Ratios of observed to expected cerebellar diameters were calculated. Sensitivity, specificity, and positive predictive values were calculated for various cutoff points and Down syndrome prevalences.ResultsCerebellar diameters in Down syndrome fetuses were smaller than in normal controls at all gestational ages (P< .005) by an average of 0.67–0.87 mm. A ratio of 0.92 for observed/expected cerebellar diameters yielded a sensitivity of 21%, specificity of 95%, and positive predictive values of 1.66% and 0.56% in populations with a risk for Down syndrome of one in 250 and one in 750, respectively.ConclusionsCerebellar hypoplasia is developmentally established and sonographically recognizable in secondtrimester Down syndrome fetuses. However, cerebellar size differences between normal and Down syndrome fetuses are too small to be clinically useful.