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Down Syndrome: Overview

What is Down syndrome?

Down syndrome is a genetic condition where an individual has three, rather than two, copies of chromosome 21. Down syndrome is the most frequently occurring chromosomal abnormality and effects one in every 691 babies born in the United States. The disorder was first identified in 1866 by John Langdon Down, a British physician, and later named after him. Today, there are more than 250,000 people living with Down syndrome in the United States.

Children who are born with Down syndrome have an increased risk for certain medical conditions such as congenital heart defects, thyroid conditions, airway and respiratory problems, vision and hearing problems, and occasionally childhood leukemia (1%). These children will also experience mild to moderate cognitive delays.

Quality educational programs, a stimulating home environment, appropriate therapies, good health care and positive support from family, friends and the community enable people with Down syndrome to develop their full potential and lead fulfilling lives.

What causes Down syndrome?

Down syndrome occurs because of the extra copy of chromosome 21, which can cause the body and brain to develop differently than a child without the syndrome.  Researchers have extensively studied the defects in chromosome 21 that cause Down syndrome. 

In 88% of cases, the extra copy of chromosome 21 is derived from the mother.  In 8% of the cases, the father provided the extra copy of chromosome 21.  In the remaining 2%, Down syndrome is due to mitotic errors, an error in cell division which occurs after fertilization when the sperm and ovum are joined. 

Who gets Down syndrome?

The incidence of Down syndrome rises with increasing maternal age.  Many specialists recommend that women who become pregnant at age 35 or older undergo testing for Down syndrome.  The likelihood that a woman under 30 who becomes pregnant will have a baby with Down syndrome is less than 1 in 1,000, but the chance of having a baby with Down syndrome increases to 1 in 400 for women who become pregnant at age 35.

A consult with a genetic counselor or geneticist is important to investigate the risk of recurrence in younger mothers. 

Helpful resources:

Global Down Syndrome Foundation
Linda Crnic Institute for Down Syndrome
Rocky Mountain Down Syndrome Association
National Down Syndrome Society
National Down Syndrome Congress
Colorado Springs Down Syndrome Association
The Arc of Colorado
Denver Adult Down Syndrome Clinic