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Down Syndrome

Medically reviewed by Last updated on Feb 21, 2022.

What is Down Syndrome?

Harvard Health Publishing

Down syndrome is a disorder caused by a problem with the chromosomes — the pieces of DNA that have the blueprint for the human body. Normally a person has two copies of each chromosome, but a person with Down syndrome has three copies of chromosome 21. The condition also is called trisomy 21.  

In a few cases, the extra copy is part of another chromosome (translocation), or found in only some of the person's cells (mosaicism). 

The extra DNA makes the physical and mental characteristics of Down syndrome, which include a small head that is flattened in the back; slanted eyes; extra skin folds at the corners of the eyes; small ears, nose and mouth; big-looking tongue; short stature; small hands and feet; and some degree of mental disability.  

Down syndrome affects an estimated 1 in 800 births. It's the most common chromosome problem seen in live births. 


In addition to the characteristic physical features and decreased mental abilities, other health problems frequently are seen in people with Down syndrome. These include: 

  • Hearing deficits 
  • Heart problems 
  • Intestinal abnormalities 
  • Eye problems 
  • Low levels of thyroid hormone 
  • Skeletal problems such as joint instability 
  • Poor weight gain in infants 
  • Kidney and urinary tract anomalies 

People with Down syndrome develop leukemia more often than those without the disorder, and they are more likely to develop infections, problems with the immune system, skin disorders and seizures. 

Infants with Down syndrome usually develop more slowly than other children of the same age, although a wide variation is seen. Language development is typically much slower, as is motor development. Their body strength may seem a little weak. For example, most toddlers walk between 12 and 14 months of age, but toddlers with Down syndrome walk between 15 and 36 months. 


Down syndrome frequently is suspected at birth based on physical appearance. The diagnosis usually is confirmed by a blood test to examine the chromosomes. Additional testing may be done, including chest X-rays, echocardiography and an electrocardiogram, to check for heart problems. Sometimes X-ray studies of the gastrointestinal tract are done as well. 

In some cases, Down syndrome is suspected during pregnancy from the results of a fetal ultrasound and blood test that measures the levels of three chemicals (a "triple-screen" test) in a pregnant woman's blood. If these results are abnormal, further tests can be done to help diagnose Down syndrome.

Expected Duration

Down syndrome continues throughout life.


There is no way to prevent Down syndrome. However, the chance of having a child with Down syndrome increases as the age of the mother increases. 

Parents who already have a child with Down syndrome are more likely to have another child with the same problem in future pregnancies. Genetic testing can help to determine the amount of risk.


There is no treatment to reverse the genetic abnormality that causes Down syndrome. However, many of the associated medical and developmental conditions can be treated to: 

  • enhance the person's quality of life 
  • improve the child's development, and  
  • increase his or her life expectancy. 

Many health care professionals may be involved in assessing and planning the course of treatment for a child with Down syndrome. Surgery may be required for cardiac or gastrointestinal problems. 

Physical therapy and integrated special education services help children with Down syndrome to make the most of their abilities and reach their potential. Children with Down syndrome usually respond very well to sensory stimulation, exercises to help their muscle control, and activities to help their mental development. School helps children with Down syndrome to learn social, academic and physical skills that may allow them to attain a very high level of functioning and independence.

When To Call a Professional

Most cases of Down syndrome are detected early in life. Call your doctor if you suspect that your child has Down syndrome that has not been diagnosed or if you have questions about your risk of having a child with Down syndrome.


The outlook for a person with Down syndrome varies with the accompanying medical and developmental conditions. The outlook continues to improve, as educators and health care professionals recognize the importance of early interventions to promote both health and development. Advances in medical treatments have greatly improved the life expectancy for people with Down syndrome, with the majority living past age 55.

Additional Info

National Down Syndrome Congress

National Down Syndrome Society

Learn more about Down Syndrome

Treatment options

Care guides

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.