[Trisomy 21 by isochromosome: a case report of true false negative of chorionic villi sampling]

We report a rare case of true false negative of chorionic villi sampling for a child with Down syndrome. A chorionic villi sampling was performed for a nuchal translucency at the first trimester. The karyotype was 46,XX for the short and the long-term culture. Because of facial dysmorphy and cardiopathy to the child, a karytoype was proposed. This postnatal karyotype showed a trisomy 21, by isochromosome 46,XX,i(21)(q10). We expose the mechanism of true false negative of chorionic villi sampling, and particularly the role of isochromosome in this case.