What is Down syndrome?

Down syndrome (also called trisomy 21) is the most common chromosomal condition in the United States. A chromosomal condition happens when there’s a problem in one or more chromosomes.

Babies with Down syndrome have an extra copy of chromosome 21, which changes how the brain and body develop. People with Down syndrome may have physical problems, learning problems and distinct physical features. Many people with Down syndrome are healthy, but some need medical care for birth defects or other health conditions. Learn more at: marchofdimes.org
Coleen  Boyle, PhD, MS
Public Health & General Preventive Medicine
Down syndrome is a condition in which a person has an extra chromosome. Chromosomes are small packages of genes in the body that determine how the fetus's body forms during pregnancy and how the body functions as it grows in the womb and after birth. Typically, a baby is born with 46 chromosomes. Babies with Down syndrome have an extra copy of one of these chromosomes, chromosome 21. A medical term for having an extra copy of a chromosome is trisomy, and Down syndrome is also referred to as trisomy 21.

As this extra copy changes how the baby’s body and brain develop, it can cause both mental and physical challenges for the baby.

The presence of the CDC logo and CDC content on this page should not be construed to imply endorsement by the US Government of any commercial products or services, or to replace the advice of a medical professional. The mark “CDC” is licensed under authority of the PHS.
Down syndrome is an abnormality of the chromosomes and the most common cause of mental retardation among infants. Down syndrome is characterized by certain medical problems such as congenital heart defects. There is a range of severity of Down syndrome, ranging from those with mild disability to severe.

Down syndrome is a chromosomal disorder that results in some degree of mental retardation. It occurs in 1 out of every 660 births and is diagnosed in utero or at birth. Symptoms vary widely from child to child.

From Good Kids, Bad Habits: The RealAge Guide to Raising Healthy Children by Jennifer Trachtenberg.

Take the RealAge Test!

Good Kids, Bad Habits: The RealAge Guide to Raising Healthy Children

More About this Book

Good Kids, Bad Habits: The RealAge Guide to Raising Healthy Children

When kids start going on TV binges or devouring fistfuls of cookies, it's easy to say, "They'll grow out of it." More likely, they're acquiring bad habits that could lead to childhood obesity and...

Down syndrome is the most frequent genetic cause for mild to moderate mental retardation and related medical problems. It is caused by a chromosomal abnormality. For an unknown reason, a change in cell growth results in 47 instead of the usual 46 chromosomes. This extra chromosome changes the orderly development of the body and brain.

This information is based on source information from KidsHealth.org.

Down syndrome is a chromosomal abnormality. A person with Down syndrome is born with more genetic material than is typical. This excess causes normal growth and development to be altered. As a result, a person with Down syndrome may have a higher risk for a number of physical and intellectual conditions, such as heart defects, hearing and vision problems, digestive issues, and cognitive delay. It is important to keep in mind, though, that not everyone with Down syndrome will experience every possible condition or limitation.

Continue Learning about Down Syndrome

Down Syndrome

Down Syndrome

Down syndrome, or trisomy 21, is caused by the presence of a third copy of chromosome 21. The risk for Down syndrome is higher in babies of mothers older than 35, who are more likely to have abnormal eggs that may lead to the cond...

ition. Common symptoms include problems in growth, hearing, vision and immunity, along with problems of the thyroid, heart, muscles and nerves. The severity of these symptoms varies from person to person.

Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.