Comparison between two methods of standardization for gestational age differences in fetal nuchal translucency measurement in first-trimester screening for trisomy 21

Our purpose was to compare two different methods of expressing nuchal translucency (NT) measurements in first trimester screening for trisomy 21: the difference in millimeters from the median of nuchal translucency (delta value: delta NT) and the multiple of the expected median (MoM). Fetal nuchal translucency was measured in 32 fetuses with trisomy 21 and in 3180 normal fetuses at 9-13 weeks' gestation. For each fetus, the measured nuchal translucency was expressed both as a delta value and MoM. The effectiveness of the MoM-Gaussian vs. the delta value method in modifying the age-specific risk for trisomy 21 was compared by using both the maternal age distribution of our study population and the age distribution of a general obstetric population. The use of the MoM-Gaussian approach led to a reduction in the false-positive rate at a given detection rate, both in the study population (by 1.2-15.2%) and in the general population (by 0.4-2.4%). Our results suggest that the use of the MoM-Gaussian method might confer a potential advantage on the screening performance of nuchal translucency in combination with maternal age by decreasing the false-positive rate. Further studies in larger unselected populations will be needed to confirm the effectiveness of this approach.