Down syndrome: sonographic sign for diagnosis in the second-trimester fetus.

In an expanded series of patients a sonographic sign was found to correlate well with the diagnosis of Down syndrome in second-trimester fetuses. The sign is characterized by soft-tissue thickening at the back of the fetal occiput. Sonographic evaluation for this sign was done in 2,121 consecutive fetuses between 15 and 20 gestational weeks of age at the time of genetic amniocentesis. The mothers were 35 years old or older or were otherwise at increased risk for fetal trisomies. Ten fetuses had Down syndrome by karyotype and four of these (40%) had thickened soft tissues (6 mm or more) at the back of the neck or occiput. When these data are combined with those in series previously reported by the authors, similarly collected consecutively, total second-trimester fetuses studied equals 3,825. Nine of the 21 (42%) Down syndrome fetuses by karyotype from this 3,825 had the positive sonographic sign described.