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Down Syndrome (Trisomy 21)

(Down's Syndrome; Trisomy G)


Nina N. Powell-Hamilton

, MD, Sidney Kimmel Medical College at Thomas Jefferson University

Reviewed/Revised Nov 2023
Topic Resources

Down syndrome is a chromosome disorder caused by an extra chromosome 21 that results in intellectual disability and physical abnormalities.

An extra chromosome, making three of the same chromosome (instead of the normal two), is called trisomy.

Trisomy 21 causes about 95% of the cases of Down syndrome. Thus, most people with Down syndrome have 47 chromosomes instead of the normal 46. However, about 4% of people with Down syndrome have 46 chromosomes, but the extra chromosome 21 is attached to another chromosome, creating an abnormal, but not extra, chromosome. This abnormal attachment is called translocation.

The extra chromosome usually comes from the mother, and a couple's risk of having a baby with an extra chromosome gradually increases with the mother's age. Yet, because most births occur to younger women, just 20% of infants with Down syndrome are born to mothers older than 35.

Women who have Down syndrome have a 50% chance of having a child with Down syndrome. However, there seems to be an increased risk of miscarriage of affected fetuses. Men with Down syndrome are usually infertile, unless they have mosaic Down syndrome. People who have mosaic Down syndrome have a mixture of two types of cells. Some of the cells contain the usual 46 chromosomes and some cells contain 47 chromosomes. The cells that have 47 chromosomes contain an extra chromosome 21.

Complications of Down syndrome

Down syndrome affects many parts of the body. Not all complications are present in each person.


The joints in the neck may be unstable, causing compression of the spinal cord, which can lead to changes in gait, use of the arms and hands, bowel or bladder function, or weakness.

Symptoms of Down Syndrome

In Down syndrome, physical and mental development is typically delayed. Children with Down syndrome are often short and are at increased risk of becoming obese.

Physical development

Infants with Down syndrome tend to be placid and passive and do not cry as often as expected. Many infants are born with heart and gastrointestinal defects, and they have somewhat limp muscles. They tend to have a small head and a face that is broad and flat with a short nose. There may be extra skin around the back of the neck (nuchal folds). However, some newborns do not have obvious characteristic facial features at birth and then develop noticeable characteristic facial features during infancy.

Typical physical features of people with Down syndrome include the following:

  • The eyes slant upward at the edges, and skin of the upper eyelid covers the inner corner of the eye (epicanthal fold).

  • The tongue is sometimes large. The larger tongue plus low facial muscle tone frequently causes children to hold their mouth open.

  • The ears are small, rounded, and set low in the head.

  • The hands are typically short and broad, with a single crease across the palm. The fingers are short, and the fifth finger, which often has two instead of three sections, curves inward.

  • The space between the first and second toes may also be widened (sandal-gap toes).

Characteristic Physical Features of Down Syndrome

Cognitive (intellectual) development

The intelligence quotient (IQ) among children with Down syndrome varies but averages about 50, compared with children in the general population, whose average IQ is 100.

Children with Down syndrome often have delays in developing motor and language skills, but this is variable.

Diagnosis of Down Syndrome

  • Before birth, ultrasonography of the fetus or blood tests of the mother

  • Before birth, chorionic villus sampling, amniocentesis, or both

  • After birth, the appearance of the infant and blood tests of the infant

Before birth, Down syndrome may be suspected based on findings detected during an ultrasound of the fetus Ultrasonography Prenatal diagnostic testing involves testing the fetus before birth (prenatally) to determine whether the fetus has certain abnormalities, including certain hereditary or spontaneous genetic... read more or based on abnormal levels of certain proteins and hormones in the mother’s blood in the first 15 to 16 weeks of pregnancy. Doctors also can do a test to find deoxyribonucleic acid (DNA) from the fetus in the mother's blood and use this DNA to determine an increased risk of Down syndrome. This test is called noninvasive prenatal screening (NIPS) or cell-free fetal DNA analysis.

Screening and diagnostic testing for Down syndrome before 20 weeks of pregnancy is recommended for all women regardless of age.

After birth, an infant with Down syndrome typically has a physical appearance that suggests the diagnosis. A doctor usually confirms the diagnosis by testing the infant’s blood.

After the diagnosis is made, doctors do other tests to screen for abnormalities associated with Down syndrome. Treating such abnormalities can often prevent them from impairing health. These tests are done at certain intervals and include the following:

Children who have neck or nerve pain, weakness, or other neurologic symptoms should have x-rays of the bony joints of the neck to check for instability. Children and adults who want to participate in the Special Olympics or other sporting events also may need to have x-rays of the bony joints of the neck.

Older adolescents and adults with Down syndrome should be screened at certain intervals for the following diseases associated with Down syndrome:

  • Diabetes

  • Underactive thyroid gland (hypothyroidism)

  • Alzheimer disease

Treatment of Down Syndrome

  • Screening for complications and associated diseases

  • Treatment of specific symptoms and problems

  • Genetic counseling

There is no cure for Down syndrome. However, some specific symptoms and problems caused by the syndrome can be treated. Doctors can surgically repair some heart and gastrointestinal defects. Other diseases (such as hypothyroidism, celiac disease, diabetes, and leukemia) are treated as needed.

Care for people with Down syndrome should also include genetic counseling for the family, social support, and educational programming appropriate for the level of intellectual functioning (see Treatment of Intellectual Disability Treatment Intellectual disability is significantly below average intellectual functioning present from birth or early infancy, causing limitations in the ability to conduct normal activities of daily... read more ). Early intervention with educational and other services improves the functioning of young children with Down syndrome.

Prognosis for Down Syndrome

The aging process seems to be accelerated, but most children with Down syndrome survive to adulthood. The average life expectancy is about 60 years, and some affected people live into their 80s. Symptoms of dementia, such as memory loss, further lowering of intellect, and personality changes, may develop at an early age. Heart abnormalities are often treatable with medications or surgery. Heart disease, infections, and leukemia account for most deaths among people with Down syndrome.

Research indicates that Black people with Down syndrome have a substantially shorter life span than White people with Down syndrome. This finding may be the result of poor access to medical, educational, and other support services.

More Information

The following English-language resources may be useful. Please note that THE MANUAL is not responsible for the content of these resources.

See the following sites for comprehensive information on everything from updates on research and funding initiatives to training for professionals and the general public, educational materials, and quick links to related topics:

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