Minor sonographic signs of trisomy 21 at 15-20 weeks' gestation in fetuses born without malformations: a prospective study.

A prospective study was performed on 2119 pregnancies that underwent genetic amniocentesis. Indications for amniocentesis were either maternal age (> or =35) or triple-test results (risk> or =1/380). The study covered a 36-month period and assessed the prevalence of minor ultrasound markers both in fetuses with Down syndrome and normal control fetuses at 15-19 week' gestation. Only fetuses with normal karyotype or trisomy 21 were considered. Six minor sonographic markers were considered: nuchal thickness, pyelectasia, femur observed/expected and humerus observed/expected ratios, bowel echogenicity, and choroid plexus cysts. One or more ultrasound soft markers were present in 23 out of 33 fetuses with Down syndrome (70%) and in 572 out of 2069 normal fetuses (28%).