Down syndrome (trisomy 21) is one of the common intellectual disability with characteristic facies and mental retardation. This syndrome typically shows marked small ears, upslanting palpebral fissures, hypotonia, flat facial profile. In general 95% of this syndrome are resulting from non-disjunctional error of chromosome 21 during gametogenesis. 2% result from Robertsonian translocation (especially 14;21) of which 50% are familial. 2% result from mosaicism. In a retrospective study with 5737 cases, ninety five percent of them had regular trisomy 21. In 4% there was a translocation mostly Robertsonian t(14;21) and t(21;21). We report here prenatal diagnosis of a fetus with trisomy 21, 46, XX, t(21;21)(p12;p12) at chorionic villus sampling. Prenatal diagnosis was done for ultrasonography demonstrated cystic hygroma. This is a de novo translocation, parents had normal karyotypes.