Risk factors associated with Down syndrome vary according to its classification. Factors that increase the risk that a baby will have trisomy 21 type Down syndrome, the most common type (95% of all cases), include:

• Having an older mother. Women who are older than 35 have an increased risk for having a child with trisomy 21 type Down syndrome. This risk increases continually with advancing age.
• Having an older father. Medical researchers are looking at the link between a man's age and the risk of having a child with Down syndrome. Early studies suggest that if a father is older than 40 and a mother is older than 35, they have an increased risk of having a child with Down syndrome.²
• Having a sibling with Down syndrome. Women who have had a child with trisomy 21 type Down syndrome have a 1-in-100 chance of having another child with the condition.³

Mosaicism is a type of Down syndrome that produces extra genetic material in some of the baby's cells, while the other cells develop normally. Mosaicism affects about 3 out of 100 people who have Down syndrome.⁴ The risk factors for mosaicism are similar to those for trisomy 21.

Translocation-type Down syndrome is the only type that is sometimes directly inherited. But the majority of translocation-type Down syndrome cases are sporadic (random) genetic mutations, with no known cause. Translocation accounts for about 4% of all cases of Down syndrome.⁵

You may be a carrier of the translocation gene if you have:

• A family history of Down syndrome. Translocation-type Down syndrome genes may have been passed on to you if other members of your family have the condition.
• Had other children with Down syndrome. Although translocation-type Down syndrome is most often a sporadic genetic mutation, if you have had a child with this type, you may be a carrier of the gene.

Questions still need to be answered about the specific risk factors for Down syndrome. So far, research has not found any environmental influences that contribute to developing the condition. Medical researchers do not know why cells sometimes divide abnormally and produce the extra genetic material that results in Down syndrome.