What is a child with Down syndrome? Down syndrome is a congenital disease, also known as trisomy 21, which is usually caused by an abnormal number of chromosomes in the fetus. Normally, human cells have 23 pairs of chromosomes, while children with Down syndrome have three pairs of chromosome 21, which is where the name comes from. Down\’s syndrome is the result of abnormal chromosome segregation in germ cells and abnormalities in early embryonic development. It is present in the cellular genetic material of the embryo. It is usually caused by problems with the mother\’s egg or the father\’s sperm during fertilization. The incidence and symptoms of children with Down syndrome. Children with Down syndrome are one of the common congenital malformations, with an incidence rate of 800%.