Down syndrome is one of the most common chromosomal abnormalities in humans. Parents of children with Down syndrome are, for the most part, genetically healthy. In the highest percentage of cases, Down syndrome is not hereditary. Only if one of the parents is identified as a carrier of a Robertsonian translocation involving chromosome 21 (which occurs in 3-4% of cases), Down syndrome can be considered an hereditary syndrome.
In families that have had children with Down syndrome, the chance of recurrence is 1 in 100.
The risk of giving birth to a child with Down syndrome increases with the age of the woman: from about 1 in 1,250 children in pregnant women aged 25, to about 1 in 100 children in pregnant women aged 40. However, most children with Down syndrome are born to women under the age of 35, because younger women have more children.