Also referred to as Down's syndrome or Down's, a genetic disorder present at birth that is caused by an abnormality in the number of chromosomes.

Down syndrome occurs with equal frequency in people of different nationalities, social backgrounds, and economic classes, averaging 1 in about every 600 births. In the United States, over 400,000 people have the condition. Women over 35 are at greater risk of bearing a child with Down syndrome than younger women. One in 1500 children with Down syndrome is born to a mother under 30; for mothers over 45, this figure rises to one in 65. Altogether, though, 80% of all Down infants are actually born to women under 35. According to recent studies, the age of the father may play a role as well. Prenatal detection of Down syndrome is possible through amniocentesis and chorionic villus sampling and is recommended for pregnant women over the age of 35.

John Langdon Haydon Down...