Randomized, opened multicentric study evaluating the Effectiveness of Pessary in the patients with a Gemellary pregnancy and a collar runs between 20 and 28 weeks of amenorrhoea To show that the use of pessary makes it possible to lengthen the duration of the pregnancy of at least 10 days in patients with a twin pregnancy and a uterine collar length < = 25 mm measured by echography between 20 and 28 SA compared to the group controls.