The trisomy 21 is the most frequent cause of handicap of genetic origin with an incidence of 1.3/1 000 births which increases with the maternal age. Several strategies of antenatal screening were developed. The High Authority of Health recently recommended to propose to the pregnant women a 1st trimester combined screening, realized between 11+0 and 13+6 weeks of amenorrhoea, associating measure of the NUCALE translucency and dosage in maternal serum : PAPP-A (Pregnancy Associated Plasma Protein A) and βhCG (free β human Chorionic 1st trimester Gonadotrophin). However, the sensibility of these screening tests is about 80 % for 5 % of positive false. The diagnosis of aneuploidy is then established on a population classified at high risk, by a foetal karyotype (requiring an invasive procedure : biopsy of trophoblast or amniocentesis). A positive screening increase maternal and medical anxiety. Furthermore, the inappropriate combination of the tests is at the origin of numerous useless invasive procedures (national rate of amniocentesis of 11 %), at risk of foetal losses (0,5 in 1 %). This inflation of invasive procedures leads to so many losses of not affected children as trisomic children and to a badly known maternal morbidity.