Preliminary normal reference values of nuchal translucency thickness in Taiwanese fetuses at 11-14 weeks of gestation.

BACKGROUND To investigate normal reference values of nuchal translucency (NT) thickness in normal Taiwanese fetuses between 11 and 14 weeks of gestation. METHODS A prospective study of ultrasound measurements of fetal NT and crown-rump length (CRL) at 11-14 weeks of gestation was conducted in 724 consecutive Taiwanese fetuses between 1998 and 2001. The relationship between NT and 5-mm intervals of the CRL of the fetus was analyzed. NT thickness was converted into multiple of median (MoM) values for the proper CRL. The estimated risk of trisomy 21 was calculated in combination with maternal age and NT MoM. RESULTS NT thickness increased with increasing CRL and gestational week in the first trimester. The mean (median) of NT thickness at 11-14 weeks was 1.56 (1.50) mm. Values of NT logMoM showed a normal Gaussian distribution with a mean of -0.0062 and standard deviation of 0.1146. The overall frequency of NT thickness of > 2.5 mm and > 3.0 mm was 1.7% (12/724) and 0.7% (5/724), respectively. There were 18 (2.5%) of 724 normal fetuses with the eseimated risk of trisomy 21, based on maternal age and NT thickness higher than 1:300. CONCLUSIONS Because of weekly variations and racial differences in NT measurements, normal reference values should be established to convert NT thickness into MoM values for calculating the estimated risk of trisomy 21 in first-trimester NT screening.