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: δNT) 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. Copyright © 1997 International Society of Ultrasound in Obstetrics and Gynecology