Trisomy 17 mosaicism in amniotic fluid cells not found at birth in blood but present in skin fibroblasts

: This article describes a case of fetal trisomy 17 mosaicism found in amniotic fluid cells in one of two bichorial biamniotic twins without any sonographic anomaly. The extra chromosome 17 was absent from cord blood cells at birth but present on karyotype and in situ hybridization in cultured fibroblasts from skin biopsy. Clinical examination showed a few mild dysmorphic features and a moderate neurological involvement which may rather be related to prematurity. It therefore seemed important to obtain the karyotype on fibroblasts when a trisomic cell line was found in amniocentesis and not confirmed on blood lymphocytes, even in the absence of dysmorphic features. This should help to differentiate a real mosaic from a mosaic restricted to extra-fetal tissues.