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Down Syndrome (Trisomy 21)

(Down's Syndrome; Trisomy G)

By Nina N. Powell-Hamilton, MD, Clinical Assistant Professor of Pediatrics ;Medical Geneticist , Sidney Kimmel Medical College at Thomas Jefferson University;Nemours/Alfred I. duPont Hospital for Children

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

  • Down syndrome is caused by an extra copy of chromosome 21.

  • Children with Down syndrome have delayed physical and mental development, specific head and facial features, and are short.

  • The diagnosis is suggested by the child’s physical appearance and is confirmed by finding an extra copy of chromosome 21 in a blood sample.

  • Most children with Down syndrome survive to adulthood.

Chromosomes are structures within cells that contain DNA and many genes. A gene is a segment of deoxyribonucleic acid (DNA) and contains the code for a specific protein that functions in one or more types of cells in the body (see Genes and Chromosomes for a discussion about genetics). Genes contain instructions that determine how the body is supposed to function.

An extra chromosome, making three of a kind, is called trisomy (see also Overview of Chromosome and Gene Disorders). The most common trisomy in a newborn is trisomy 21 (three copies of chromosome 21, which is the smallest human chromosome). It is possible for an embryo to have trisomy of any chromosome, however, an extra copy of a larger chromosome is more likely to end in miscarriage or stillbirth. Trisomy 21 causes about 95% of the cases of Down syndrome. Thus, most people with Down syndrome have 47 chromosomes. The remaining 5% of people with Down syndrome have 46 chromosomes, but the extra chromosome 21 is incorrectly joined with another chromosome (called translocation), creating an abnormal, but not extra, chromosome.

The extra chromosome rarely comes from the father. However, older mothers, especially those older than 35, more commonly contribute the extra chromosome. 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, many affected fetuses abort spontaneously. 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.

Some Complications of Down Syndrome

Body System



Acute lymphocytic leukemia (ALL) or acute myelogenous leukemia (AML)

Low platelet count (thrombocytopenia)

High red blood cell count in the newborn (polycythemia in the newborn)


Intellectual disability (mild to severe)

Delayed speech and motor skills

Digestive tract

Malformed intestines

Endocrine system

Eyes and ears

Cataracts, glaucoma, crossed eyes (strabismus)



Muscles and bones

Unstable connections between the first 2 bones in the neck

Loose joints

*Not all complications are present in each person.


In Down syndrome, physical and mental development is typically delayed.

Physical development

Infants 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 slanting eyes and a short nose. However, some newborns appear normal at birth and then develop characteristic facial features during infancy. The tongue is sometimes large. The larger tongue plus low facial muscle tone frequently causes children to hold their mouth open. There may be extra skin around the back of the neck. The ears are small, rounded, and set low in the head.

The hands are often 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. Children with Down syndrome are short and are at increased risk of becoming obese.

About 50% of children with Down syndrome are born with heart defects. About 5% of children have gastrointestinal problems. Hirschsprung disease and celiac disease also are more common among affected children. They have hearing loss and are prone to recurring ear infections. They are also prone to vision problems because of problems in their corneas and lenses. 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. Many people with Down syndrome develop thyroid disease (such as hypothyroidism) and diabetes. They are also at a higher risk of developing infections and leukemia.

Mental development

The intelligence quotient (IQ) among children with Down syndrome varies but averages about 50, compared with normal children, whose average IQ is 100. Children with Down syndrome have better visual motor skills (such as drawing) than skills that require listening. Thus, their language skills typically develop slowly. Behavior suggestive of attention-deficit/hyperactivity disorder is often seen in childhood. Children with Down syndrome are at greater risk of autistic behavior, especially those with severe intellectual disability. Depression is common among adults (see Depression) and children (see Depression and Mood Dysregulation Disorder in Children and Adolescents). Early intervention with educational and other services improves the functioning of young children with Down syndrome.

Did You Know...

  • Only 20% of infants with Down syndrome are born to mothers older than 35.


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

  • 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 or based on abnormal levels of certain proteins found in the mother’s blood in the first 15 to 16 weeks of pregnancy. Doctors can sometimes find deoxyribonucleic acid (DNA) from the fetus in the mother's blood and use this DNA to determine an increased risk of Down syndrome. If doctors suspect Down syndrome based on these tests, they often confirm the diagnosis using chorionic villus sampling, amniocentesis, or both (see Testing for chromosome and gene abnormalities). Screening for Down syndrome before 20 weeks of pregnancy is recommended for all women regardless of age.

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

After the diagnosis is made, doctors use tests, such as ultrasonography of the heart and blood tests, along with examinations by specialists, to screen for abnormalities associated with Down syndrome. Treating such abnormalities can often prevent them from impairing health. Thus, these children should have regular screenings for thyroid disease, vision problems, and hearing problems.

Height, weight, and head circumference measurements are plotted at each well-child visit using a growth chart created specifically for children with Down syndrome. Children are also evaluated for obstructive sleep apnea. 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.


Down syndrome has a better prognosis in comparison to other disorders caused by an extra chromosome, such as trisomy 18 or trisomy 13.

The aging process seems to be accelerated, but most children with Down syndrome survive to adulthood. The average age at death is 49. Recently, however, some people have been living into their 70s. Symptoms of Alzheimer-like dementia, such as memory loss, further lowering of intellect, and personality changes, may develop at an early age. Heart abnormalities are often treatable with drugs or surgery. Heart disease and leukemia account for most deaths among children with Down syndrome.

Recent findings indicate that blacks with Down syndrome have a substantially shorter life span than whites. This finding may be the result of poor access to medical, educational, and other support services.


  • 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. Doctors give thyroid hormone replacement therapy to people with hypothyroidism.

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 intellectual disability treatment).

More Information

  • National Down Syndrome Congress (NDSC)

  • National Down Syndrome Society (NDSS)

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