Cut-off criteria for second-trimester nuchal skinfold thickness for prenatal detection of Down syndrome in a Thai population

Objective: To evaluate the use of progressive cut-offs for nuchal skinfold thickness with advancing gestational age and the commonly applied cut-off m ethod (greater than or equal to 6 mm) for prenatal detection of Down syndro me in a Thai population. Method: A prospective study was performed by exper ienced perinatologists on 2150 women undergoing second-trimester amniocente sis for the indications of advanced maternal age and past history of chromo somal abnormality. Reference ranges were established for nuchal skinfold th ickness from the 16th to the 24th week, using either gestational-specific c entiles or the parametric method. Assaying different cut-off criteria for b oth centile and the parametric methods were calculated and then compared wi th the commonly applied cut-off level (greater than or equal to 6 mm.). Res ults: There were 2114 chromosomally normal pregnancies, 19 fetuses with Dow n syndrome (1:113), and 17 other chromosome abnormalities. In fetuses with normal karyotype the nuchal skinfold thickness increased with advancing ges tational age [NF (mm) = -0.502 + 0.212 GA (week), r = 0.36, P < 0.001]. The sensitivities of an abnormal nuchal skinfold thickness using different cut -off criteria for detecting Down syndrome were low (5.3-26.3%) with the fal se positive rates ranging from 2.5 to 16.5%. Conclusions: In this study, me asurement of second-trimester nuchal skinfold thickness was a poor and unre liable screening test for fetal Down syndrome in a Thai population. (C) 199 9 International Federation of Gynecology and Obstetrics.