Assessment of first-trimester nuchal translucency by daily reference intervals

Increased fetal nuchal translucency measured in the first trimester is associated with an elevated frequency of chromosomal abnormalities, yet no reference intervals for transvaginal sonographic measurements of fetal nuchal translucency in that period of development have been established. The aim of the present cross-sectional study was to construct appropriate gestational age-specific reference intervals for nuchal translucency. Transvaginal ultrasound was used in 180 normal pregnant women between 9 and 14 weeks of gestation. Reference intervals were constructed by the parametric method. These reference intervals were then compared to the traditional threshold value of 3 mm, when applied to 287 women scheduled for amniocentesis or chorionic villus sampling (CVS). Both methods had the same sensitivity, 85.7%, and negative predictive value, 99.6%, in predicting fetal chromosomal abnormalities. However, the specificity of the gestational age-related reference intervals tended to be higher than that of the 3-mm threshold: 94.6% vs. 87.9%. The positive predictive value of the former method was higher than that of the 3-mm threshold: 28.6% vs. 15%. The increased specificity and positive predictive value of these reference intervals are of particular importance when applied to women under the age of 35 years. In those women, the finding of an abnormally thick nuchal translucency can lead to the performance of amniocentesis or CVS. Therefore, using these reference intervals instead of the traditional 3-mm threshold may save a significant number of unnecessary invasive procedures.