To the Editor: The article by Benacerraf et al. (Nov. 26 issue) 1 suggests that a shortened femur and a thickened nuchal fold are valuable sonographic signs in the detection of Down's syndrome in the second trimester of pregnancy. The suggestion that routine sonography can be used to identify the subpopulation at high risk for Down's syndrome has serious implications, which prompted us to review our data. The records of the UCLA Prenatal Diagnosis Center revealed that between 1981 and the present, 28 fetuses were identified by amniocentesis as having Down's syndrome (trisomy-21 karyotype). We identified 19 patients with Down's syndrome. © 1988, Massachusetts Medical Society. All rights reserved.