Down syndrome is the name for the pattern of physical features and disorders that usually occur from an extra 21st chromosome. Chromosomes are the materials that store people's genetic information.

What is going on in the body?

Because people who have Down syndrome have an extra chromosome, the body's natural balance is upset. This upset in balance leads to various birth defects and problems of growth and development. Many of the birth defects in people who have Down syndrome are like those seen in other children. People with Down syndrome simply are likely to have birth defects more often than others.

What are the causes and risks of the condition?

Down syndrome is usually caused by an extra 21st chromosome. Most people have 46 chromosomes in each cell of their bodies. These chromosomes exist in pairs, for a total of 23 pairs. The pair associated with Down syndrome is called pair 21. Why an extra copy of this chromosome causes the features of the Down syndrome is not known.

The genetic material on the 21st chromosome pair directs the formation of compounds that the body needs to perform various functions and for normal development. The overload of information caused by the extra copy leads to the Down syndrome. Scientists have recently identified 99.7% of the genes on this chromosome. This knowledge may lead to a better understanding of the syndrome and, perhaps, even treatments for some of the effects.

Down syndrome occurs in roughly 1 of every 750 births. There is a strong link between the risk of Down syndrome and the age of the mother. The risk of having a child with Down syndrome is less than 1 in 2,500 among young women. This risk increases to roughly 1 in 350 when women reach 35 years of age. After age 45, the risk is roughly 1 in 25.