Cells normally carry two carry two matched sets of twenty-three chromosomes for a total of forty-six chromosomes. One set of twenty-three chromosomes comes from each parent. Down's syndrome occurs when one chromosome pair is damaged. That pair is designated as chromosome #21.
As a woman grows older, she faces a greater risk of having a baby with Down's syndrome. For example, a woman in her twenties faces a risk of about 1 in 4,000. By the age of thirty-five, that risk has increased to 1 in 400 and, by the age of forty-five, to 1 in 35.
Men with Down's syndrome appear to be uniformly sterile. That is, they are unable to have children. Women with Down's syndrome, however, are usually able to have babies. About half of these babies will also be born with Down's syndrome.
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