Comparison of modes of ascertainment for mosaic vs complete trisomy 21

Objective We sought to compare the indications for amniocentesis leading to the detection of either mosaicism of trisomy 21 (mosaic-T21) or complete trisomy 21 (T21). Study Design A retrospective review of a large amniocentesis database (n = 494,163) was conducted. All specimens with mosaic-T21 (n = 124) were compared with a maternal age–matched group of T21 fetuses (n = 496). Samples with normal karyotypes were matched for maternal age and served as normal controls (n = 496). The χ2 testing was used for statistical analysis. Results The presence of an abnormal first-trimester screen, abnormal sonographic findings, and specifically the single sonographic abnormalities of either a cystic hygroma or a cardiac anomaly were significantly less common in the mosaic-T21 as compared with the T21 group. There were no such differences between the mosaic-T21 and the normal control group. Conclusion Fetuses with mosaic-T21, similar to those with normal karyotype, do not present with the same abnormal screening tests as fetuses with T21.