Thickened nuchal fold in fetuses not at risk for aneuploidy.

Many studies have shown that a thickened nuchal fold in a second-trimester fetus is a sonographic sign suggestive of a high risk for Down syndrome. These series have included fetuses already at risk for aneuploidy because of advanced maternal age or abnormal maternal serum alpha-fetoprotein (AFP) levels. The authors describe the sonographic and karyotypic findings in 42 women younger than age 35 years whose fetuses were noted to have a thickened nuchal fold. Forty fetuses were not at risk for Down syndrome. Fourteen of the 42 fetuses (33%) had abnormal karyotypes; 11 had Down syndrome and one each had XXX, XO, and XYY. If the five cases with low maternal serum AFP levels are excluded, nine of the 37 fetuses (24%) with thickened nuchal folds had abnormal karyotypes. Amniocentesis would have been performed in one of those nine fetuses because of hydrocephalus. Thus, eight of 36 fetuses (22%) with thickened nuchal folds and no other indications for amniocentesis had abnormal karyotypes. When a thickened nuchal fold is present, the yield of abnormal karyotype is high (22%), making it possible to identify patients who would benefit from cytogenetic evaluation but who would not otherwise be candidates.