Method Two public centres of antenatal screening were established to answer specifically the recommendations of the High Authority of Health and have the capacity to make 20 000 acts of screenings a year. All the women presenting a high risk (> 1/250) of aneuploidy and the candidates to an invasive procedure, will see suggesting participating in the study. The included patients will have a maternal blood sample before the invasive test. This taking will be put in on hold in-80°C after centrifugation following the recommended protocol. At the end of inclusion, the technic of analysis of the circulating DNA will be realized then blind by the laboratory on all the diagnosed cases (by usual invasive examination, or at the conclusion of the pregnancy) of trisomy 21 and on control representative of all the spectre of the levels of risk. A study of reproducibility of the quantification of the DNA of the chromosome 21 will be realized.