Bedside estimation of Down syndrome risk from second-trimester ultrasound prenasal thickness

Objectives: To construct tables for 'bedside' estimation of Down syndrome risk based on maternal age and ultrasound prenasal thickness (PT) measurements. Methods: Likelihood ratios were calculated using a log Gaussian model of the PT distribution in multiples of the gestational age-specific median (MoM). The model parameters were derived from 80 Down syndrome and 850 unaffected pregnancies scanned at 14-27 weeks; these data had been published previously, in three series, except for 18 Down syndrome and 119 affected pregnancies. The means were estimated as the median, and the SDs as the 10^th-90 ^th range divided by 2.563. Results: A log Gaussian model fitted well the distribution of PT values in Down syndrome and unaffected pregnancies with medians of 1.31 MoM and 1.01 MoM, and log₁₀ SDs of 0.075 and 0.082, respectively. Conclusions: The tables provide a simple 'bedside' estimation of Down syndrome risk without the need for computerized software or complicated calculations. More prospective data on PT in combination with other first- and second-trimester screening markers are needed.