 The instigators have planned to use the ISET method in a blind study including 100 cases of trisomy 21 and 300 control cases with normal caryotype. This study will allow to obtain results with sensitivity higher than 97 and specificity higher than 99 IC 95 70-100. The validation will be obtained by the opening of the blind study and the comparison of results obtained by the invasive method amniocentesis and the non-invasive method. 