Down syndrome is also known as "Trisomy 21" which is a genetic disorder caused by the presence of extra genetic material from part or all of an extra 21st chromosome.
Down syndrome is named after John Langdon Down, the British doctor who described it in 1866. The extra genetic material causes delays in the way a child develops, and often leads to mental retardation. In general, symptoms of Down syndrome can vary widely from child to child. While some kids with Down syndrome need a lot of medical attention, others lead very healthy and independent lives.
Normally, at the time of conception a baby inherits genetic information from its parents in the form of 46 chromosomes: 23 from the mother and 23 from the father. In most cases of Down syndrome, however, a child gets an extra chromosome — for a total of 47 chromosomes instead of 46. It's this extra genetic material that causes the physical and cognitive delays associated with Down syndrome.
Although no one knows for sure why Down syndrome occurs and there's no way to prevent the chromosomal error that causes it, scientists do know that as women get older they have a significantly higher risk of having a child with the condition. Down syndrome affects about 1 in every 800 babies born, with a lower risk in younger women and a higher risk in older women. At age 30, for example, a woman has less than a 1 in 1,000 chance of conceiving a child with Down syndrome. Those odds increase to 1 in 400 by age 35. By 42, it jumps to about 1 in 60.
Risk table from: From Hook EB. JAMA 249: 2034-2038, 1983