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Understanding Down Syndrome -- the Basics

What Is Down Syndrome?

Down syndrome is one of the most common birth defects. Usually, children born with the condition have some degree of mental retardation, as well as characteristic physical features. Many of these children also have other health problems.

Each year in the U.S., approximately one in every 800 to 1,000 newborns has Down syndrome. This results in approximately 5,000 children born with Down syndrome each year. In the U.S. today, Down syndrome affects approximately 350,000 people. As many as 80% of adults with this condition reach age 55, and many live longer.

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The most common form of Down syndrome is often called "trisomy 21" because individuals with this condition have three copies of the 21st chromosome.

What Causes Down Syndrome?

Normally, each cell in the human body contains 23 pairs of chromosomes, which contain the genetic material that determines all our inherited characteristics. We receive half of each chromosome pair from our mother and the other half from our father. Individuals with the most common form of Down syndrome, trisomy 21, have an extra 21st chromosome.

No one knows exactly why this chromosomal error occurs, but it does appear to be related to the age of the mother. At age 25, a woman has a one in 1,250 risk for having a child with Down syndrome. The risk increases to one in 952 at age 30, to one in 378 at age 35, to one in 106 at age 40 and one in 35 at age 45. However, 80% of children born with Down syndrome are born to mothers under the age of 35. This is because most babies, in general, are born to younger women.

WebMD Medical Reference

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