What Is Down Syndrome?

You may hear during your pregnancy that your child is likely to have Down syndrome. But exactly what is Down syndrome? Here's what you should know.

What is Down syndrome?

In 1866, the English physician John Langdon Down published a description of the traits of Down syndrome. Almost a hundred years later, in 1959, we discovered that people with Down syndrome have an extra chromosome.

Every human cell contains a nucleus carrying our genetic code, usually with 23 chromosomes. In people with Down Syndrome, errors in conception affect chromosome 21. In most cases, all of the cells contain an extra copy of chromosome 21. Sometimes the extra chromosome attaches to another chromosome. Some cells might have an extra chromosome and others not.

We now know that chromosome 21 contains about 329 genes, which govern the characteristics of people with Down syndrome.

About one in every 700 babies in the United States are born with this condition, some 6,000 children a year.

Down syndrome in most people ranges from very mild to severe. Most people with Down syndrome have cognitive delays that are mild to moderate.

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What causes Down syndrome?

At this time, researchers don’t know why the error occurs. The extra chromosomal material usually comes from the mother — about 5 percent of cases can be traced to the father. If you are a 35-year-old woman, you have a one in 350 chance of conceiving a child with Down syndrome. The risk goes up with your age, hitting one in 100 at 40 and one in 30 at 45.

Most cases of Down syndrome are not caused by a condition you inherited from your own parents.

Many prenatal screening tests are now available that can estimate the chances that your fetus has Down syndrome. Serum screening looks at substances in the blood of the mother and, together with her age, estimates the chance of a Down syndrome child. You might also have a detailed sonogram to check for signs that suggest Down syndrome. Other screens check for material from the fetus in the mother’s blood. If you suspect Down syndrome, you can go further and have a more conclusive test: chorionic villus sampling (CVS) in the first trimester and amniocentesis in the second.

At birth, a baby with Down syndrome may have a deep crease across the palm of the hand, a slightly flattened face, and an upward slant to the eyes. Your doctors may then draw a blood sample from the baby, to conduct tests to confirm the diagnosis.

People with Down syndrome are now living to age 60 or even older, and they are more likely to be part of community organizations, including school and work.

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