Bedside estimation of down syndrome risk during first-trimester ultrasound screening

Abstract

Objective: To construct tables for 'bedside' estimation of Down syndrome risk based on maternal age and nuchal translucency measurements. Methods: Likelihood ratios were calculated using the log multiple of median Gaussian model. The parameters for the model (mean and standard deviation) were derived from 5560 normal and 51 Down syndrome-affected pregnancies scanned during the first trimester in three different centers. Equations for calculating maternal background risk and median values were obtained from previous reports. The results were compared to two modalities using the log Gaussian model and software that uses the delta-value model. Results: The distribution fitted the data well, and the parameters obtained in the study group for the log multiple of median model were a mean of 0 and a standard deviation of 0.12356 among normal pregnancies and a mean of 0.305312 and a standard deviation of 0.240337 among Down syndrome-affected ones. The likelihood ratios obtained for the various combinations of fetal crown-rump lengths and nuchal translucency measurements were comparable to other modalities reported earlier. Conclusions: The results of the current study provide useful tables for simple and accurate 'bedside' estimation of Down syndrome risk without the need for computerized software or complicated calculations.