What causes Down syndrome to occur?

Down syndrome is usually caused by an error in cell division called nondisjunction. It is not known why this occurs, however it is known that the error occurs at conception and is not related to anything the mother did or did not do during pregnancy. It has been known for some time that the incidence of Down syndrome increases with advancing maternal age. However, 80% of children with Down syndrome are born to women under 35 years of age.

The addition of the partial or full copy of the 21st chromosome which is the cause of Down syndrome can come from either the father or the mother.