Soft tissue nuchal fold in the second-trimester fetus: standards for normal measurements compared with those in Down syndrome.

There are several reports describing the sonographic sign of a thickened nuchal fold for the prenatal sonographic detection of Down syndrome in the second trimester. We prospectively obtained normative data on the nuchal folds of 303 consecutive normal fetuses undergoing genetic amniocentesis between 15 and 20 weeks' gestation. The data show that the width of the nuchal fold was consistently between 1 and 5 mm regardless of gestational age (15 to 20 weeks).