SCREENING FOR DOWN-SYNDROME DURING THE FIRST AND 2ND TRIMESTERS - IMPACT OF RISK-ESTIMATION PARAMETERS

Objectives: To evaluate impact of risk estimation parameters for scree ning for Down Syndrome during the first and second trimesters. Methods : We prospectively examined for their performance in the prenatal pred iction of trisomy 21, alphafetoprotein (AFP), unconjugated estriol (uE 3), total human chorionic gonadotropin (hCG), and its free subunits (f ree alpha-hCG, free beta-hCG) at both the first and second trimesters, and the impact of three sets of published risk estimation parameters. A total of 14,612 pregnancies were studied. All Down syndrome specime ns (12 and 11 cases for first and second trimesters, respectively) and a sample of the unaffected pregnancies were analyzed. Results: The me dian multiple of median (MoM) for total hCG was lower in the first tri mester (1.83 vs. 2.01 in the second trimester) but no loss in discrimi native power was observed if the lower variability of the results in t he first trimester is taken into account (interquartile range of 0.251 vs. 0.338). The choice of distribution parameters did not alter signi ficantly the detection rates for the various combinations of markers ( p > 0.05). False positive rates were affected significantly however an d for the combination AFP-uE3-free beta-hCG they varied from 14.6% to 22.6% (p < 0.001). Conclusions: Our results suggest that specific dist ribution parameters would be necessary to account for the lower variab ility of the markers in the first trimester and the peculiarity of the total hCG assay we used.