The efficacy of fetal sonographic biometry in Down syndrome screening

Sonographic biometry has been proposed as a second-trimester Down syndrome screening modality. Approaches have relied on the apparent "shortened" femur length of fetuses with Down syndrome. Unfortunately, significant intercenter variation has been reported in the magnitude of this femur length reduction. In an effort to overcome many of these potential biases and better estimate the magnitude of femur length shortening in fetuses with Down syndrome, a retrospective review of femur length differences between 16 Down syndrome and 194 control fetuses was carried out. All scans were performed by one examiner who used the same equipment and measurement technique. A significant reduction in the observed to expected femur length ratio for a given biparietal diameter was identified in the Down versus control fetuses (0.9574 95% confidence interval 0.9197, 0.9952 versus 0.9999 95% confidence interval 0.9913, 1.0086) (p less than 0.008). However, the magnitude of this reduction was not sufficient to permit the use of this sonographic approach as an isolated marker for fetal Down syndrome.