Trisomy 21 is the most common genetic disorder in the world. Its frequency is about 1 out of 600 births nationwide. Down syndrome is caused, in general cases, by the presence of an entire chromosome 21. Medical explorations carried out on patients with trisomy 21 allowed a better knowledge of this syndrome pathophysiology, which helped a lot in caring about patients with this disorder. Clinical signs of trisomy 21 are mostly dominated by intellectual disabilities of different degrees, malformations in different organs and viscera, as well as some disruptions of the endocrine system and immunity. Numerous therapeutic tests were carried out on patients. The results are mixed, but research studies continue other tests. An evaluation of the risk of occurrence is presently possible at the first weeks of pregnancy (7-8 weeks). The development of researches enabled at the same time to improve life conditions of persons with trisomy 21 and proposed a pertinent risk assessment.