Down Syndrome (Trisomy 21) Data

A birth defect is a problem that happens while the baby is developing in the mother's body.  Most birth defects happen during the first three months of pregnancy.  A birth defect may affect how the body looks, works, or both.

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Down syndrome (Trisomy 21) is one type of birth defect.  Review the FAQs below for more information about Down syndrome (Trisomy 21).

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What is Down syndrome?

Down syndrome is a condition in which a baby is born with an extra chromosome. Chromosomes are small packages of genes in the body. They determine how a baby’s body forms during pregnancy and how the baby’s body will function after birth.  Normally, a baby is born with 46 chromosomes. Babies born with Down syndrome have an extra copy of one of these chromosomes. This extra copy changes the body and brain’s normal development and causes mental and physical problems for the baby.

Even though people with Down syndrome might have some common physical and mental features, symptoms of Down syndrome can range from mild to severe. Usually, mental development and physical development are slower in people with Down syndrome than in those without it.

CDC estimates that each year about 3,357 babies, or 13 of every 10,000 babies, in the United States are born with Down syndrome.  Learn more about Down syndrome from the CDC's Facts about Down Syndrome page.

What problems do children with Down syndrome have?

Babies and adults with Down syndrome can have physical problems, as well as intellectual disabilities. Every baby born with Down syndrome is different. In addition to the physical signs, some might have major birth defects or other medical problems. However, many people with Down syndrome live happy, productive lives well into adulthood.  Learn more on the CDC's Facts about Down Syndrome page.

What causes Down syndrome?

Researchers know having an extra copy of chromosome 21 causes Down syndrome, but no one knows for sure why Down syndrome occurs.  Learn more about causes and risk factors on the CDC's Facts about Down Syndrome page.

Is Down syndrome preventable?

There is no known way to prevent Down syndrome. However, infants and children with Down syndrome often will benefit from special programs that help to improve their physical and mental functions. These include speech therapy, occupational therapy, and exercises for physical coordination. Children with Down syndrome usually need extra help or attention in school.  Learn more on CDC's Facts about Down Syndrome page.

Where can I learn more about Down syndrome?

Last Revised: November 12, 2018