Down syndrome is caused by an extra copy of chromosome 21. Normally, a human has 46 chromosomes in every cell of the body. Someone with Down syndrome has a total of 47 chromosomes.
This leads to problems in the development of the body and brain. It is not known why Down syndrome occurs. The chances of having a baby with Down syndrome increase with the age of the mother.
In 96% of people with Down syndrome, the cause is not hereditary.