先天愚型是一种先天性大脑发育障碍疾病。以特殊容貌,智力低下,常伴有多种先天性畸形为特征。发病率与母亲生育时的年龄明显相关,如母25～29岁生育者约为1:1350,母35～39岁生育者约为1:260,母45岁以上生育者则可高达1:50。现已查明,本病是一种与遗传有关的疾病,病因是常染色体异常,绝大多数是多了一条第21号染色体,核型为47XX(XY),+21,故又称为染色体21-三体综合征。 Down’s syndrome is a congenital brain developmental disorder. With a special appearance, mental retardation, often accompanied by a variety of congenital malformations. The incidence was significantly related to the age of the mother when she was 25 to 29 years old, with a birth weight of 1 to 1350, a mother of 35 to 39 years old with a birth weight of 1 to 260, and a mother with a child aged 45 years and over . Has been identified, the disease is a genetic disease, the etiology is an autosomal abnormalities, the vast majority are more than a chromosome 21, karyotype 47XX (XY), +21, it is also known as the chromosome 21-trisomy syndrome.