Down Syndrome Facts

About the Most Common Genetic Disorder

Hispanic toddler with Down syndrome laughing
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Down syndrome is a genetic disorder caused by a defect in chromosome 21 that occurs due to abnormal cell division. The disorder causes intellectual and developmental problems that vary in severity. It can also cause major health problems such as heart defects.

There are approximately 400,000 Americans with Down syndrome. According to the National Down Syndrome Society, 1 in every 691 babies in the United States is born with Down syndrome.

That's about 6,000 babies born with Down syndrome each year. These quick facts will give you a better understanding of Down syndrome, the most common genetic chromosomal disorder.


Human cells typically contain 23 pairs of chromosomes. Down syndrome results when chromosome 21 produces extra genetic material. There are three different genetic variations of Down syndrome:

  • Trisomy 21. This is the most common chromosomal abnormality seen in children. It occurs when the child has three copies of chromosome 21 in his or her cells instead of two. It accounts for 95 percent of Down syndrome cases. The extra chromosome comes from the mother 88 percent of the time and from the father 12 percent of the time.
  • Mosaic Down syndrome. This rare form of Down syndrome accounts for 2 to 3 percent of cases. Some, not all, cells have an extra copy of chromosome 21.
  • Translocation Down syndrome. Chromosome 21 attaches to another chromosome, either before or at conception. Approximately 75 percent of these unbalanced translocations are de novo, meaning they happen by chance. 25 percent of translocations are actually inherited from a parent. The parent is known as a "balanced carrier," meaning they have rearranged genetic material, not extra genetic material. A balanced carrier does not show any signs of Down syndrome, but they are able to pass it on to children.

    All three types of Down syndrome are genetic conditions, but only 1 percent of Down syndrome cases are actually passed from parent to child through genes.

    Risk Factors

    Some parents pose an increased risk of conceiving a child with Down syndrome. Age is one factor. The chances of having a child with Down syndrome increase as a woman ages.

    For a 35-year-old woman, the risk is 1 in 385. By age 40, the risk is 1 in 106. By 45, the risk in 1 in 30. 80 to 85 percent of children with Down syndrome are born to women under 35.

    Additionally, a woman who has one child with Down syndrome is more likely to have another child with Down syndrome. Men and women are also able to pass the disorder on to children through translocation if they are balanced carriers.

    Life Expectancy

    In recent years the life expectancy for people with Down syndrome has vastly improved. In 1983 the life expectancy was 25; today it is 60. Approximately 25 percent of conceptions with trisomy 21 end in miscarriage or stillbirth. 85 percent of infants with Down syndrome survive to 1 year of age.


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