 Current prenatal screening methods blood markers and ultrasound for trisomy 21 Down syndrome detect about 90 of cases and have a false positive rate of 90 . The results of these tests are expressed in risks for trisomy 21, the threshold being in France at 1250. Women exhibiting a higher risk are offered to undergo invasive diagnostic testing, either by amniocentesis or chorionic villus sampling. However, these invasive diagnostic methods are associated with a considerable risk of miscarriage 1-3 , and thus underline the importance to develop a safe and non-invasive prenatal diagnostic test for trisomy 21. The investigators have planned to assess the clinical impact of a non-invasive prenatal method to detect Trisomy 21 through genetic analysis of circulating trophoblastic cells. 