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 DS need a lot of medical attention, others lead very healthy and independent lives.
Although no one knows for sure why DS 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 DS. 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