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 largest percentage of cases, Down syndrome is not inherited. 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 inherited syndrome.
In families that have had children with Down syndrome, the probability of recurrence is 1:100.
The risk of giving birth to a child with Down syndrome increases with the age of the woman - from about 1 in 1250 children in 25-year-old pregnant women, to about 1 in 100 children in 40-year-old pregnant women. However, most children with Down syndrome are born to women under the age of 35 because younger women give more births.