Objective: The purpose of our study was to determine whether the frontal lobe is significantly smaller than normal in the Down syndrome fetus in midtrimester.
Study design: Frontothalamic distance, measured from the inner table of the frontal bone to the posterior thalamus, and frontothalamic distance/biparietal diameter ratio were compared in 125 normal and 19 Down syndrome fetuses between 16 and 21 weeks' gestation.
Results: In the Down syndrome group 52% had frontothalamic distance < 10th percentile. When the frontothalamic distance and the frontothalamic distance/biparietal diameter ratio were expressed as multiples of the normal median to eliminate variation caused by gestational age, the mean value of multiples of the median in Down syndrome fetuses was compared with the mean value of multiples of the median in normal fetuses and was found to be significantly smaller (p < 0.0019 and p < 0.0177, respectively). When an observed-to-expected frontothalamic distance ratio of < or = 0.84 is used as a cutoff to screen for Down syndrome, sensitivity, specificity, and positive predictive values of 21.2%, 95.2%, and 1.2%, respectively, are achieved in a population with a 1:270 risk.
Conclusion: Frontal lobe dimension is significantly shortened in Down syndrome fetuses.