Down syndrome is caused by an extra chromosome 21.
Children with Down syndrome have delayed physical and mental development, specific head and facial features, and are often short.
Before birth, Down syndrome may be suspected based on ultrasonography or tests on the mother's blood and confirmed using chorionic villus sampling Chorionic Villus Sampling 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 and/or amniocentesis Amniocentesis 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 .
After birth, the diagnosis is suggested by the child’s physical appearance and is confirmed by finding an extra chromosome 21, usually by testing a blood sample.
Most children with Down syndrome survive to adulthood.
There is no cure for Down syndrome, but some specific symptoms and problems caused by the syndrome can be treated.
(See also Overview of Chromosomal Disorders Overview of Chromosome and Gene Disorders Chromosomes are structures within cells that contain a person's genes. A gene is a segment of deoxyribonucleic acid ( DNA) and contains the code for a specific protein that functions in one... read more .)
Chromosomes Chromosomes Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body or the code for functional ribonucleic... read more 
are structures within cells that contain DNA DNA Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body or the code for functional ribonucleic... read more 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 Genes and Chromosomes Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body or the code for functional ribonucleic... read more for a discussion about genetics). Genes contain instructions that determine how the body is supposed to look and function.
An extra chromosome, making three of the same chromosome (instead of the normal two), is called trisomy ( see also Overview of Chromosome and Gene Disorders Overview of Chromosome and Gene Disorders Chromosomes are structures within cells that contain a person's genes. A gene is a segment of deoxyribonucleic acid ( DNA) and contains the code for a specific protein that functions in one... read more ). 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 of one of the larger chromosomes is more likely to end in miscarriage Miscarriage A miscarriage is the loss of a fetus before 20 weeks of pregnancy. Miscarriages may occur because of a problem in the fetus (such as a genetic disorder or birth defect) or in the woman (such... read more or stillbirth Stillbirth Stillbirth is death of a fetus after 20 weeks of pregnancy. Stillbirth may result from a problem in the woman, placenta, or fetus. Doctors do blood tests to try to identify the cause of a stillbirth... read more . 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. About 3% 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, 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, many affected fetuses are miscarried 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.
Symptoms of Down Syndrome
In Down syndrome, physical and mental development is typically delayed.
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 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 (nuchal folds). 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). Children with Down syndrome are often short and are at increased risk of becoming obese.
About 50% of children with Down syndrome are born with heart defects Overview of Heart Defects About one in 100 babies is born with a heart defect. Some are severe, but many are not. Defects may involve abnormal formation of the heart's walls or valves or of the blood vessels that enter... read more , the most common of which are ventricular septal defect Atrial and Ventricular Septal Defects Atrial and ventricular septal defects are holes in the walls (septa) that separate the heart into the left and right sides. Holes can be present in the walls of the heart between the upper heart... read more 
and atrioventricular septal defect Atrioventricular Septal Defects Atrioventricular (AV) septal defect is a combination of heart defects. These include a hole in the wall that separates the upper chambers of the heart ( atrial septal defect), a single valve... read more . About 5% of children have gastrointestinal problems. Hirschsprung disease Hirschsprung Disease Hirschsprung disease is a birth defect in which a section of the large intestine is missing the nerve network that controls the intestine's rhythmic contractions. Symptoms of intestinal obstruction... read more and celiac disease Celiac Disease Celiac disease is a hereditary intolerance to gluten (a protein found in wheat, barley, and rye) that causes characteristic changes in the lining of the small intestine, resulting in malabsorption... read more 
also are more common among affected children. They may have hearing loss and are prone to recurring ear infections. They are also prone to vision problems and may have cataracts. 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 Hypothyroidism Hypothyroidism is underactivity of the thyroid gland that leads to inadequate production of thyroid hormones and a slowing of vital body functions. Facial expressions become dull, the voice... read more 
) and diabetes. They also have a higher risk of developing infections and leukemia and a much higher risk of developing obstructive sleep apnea Obstructive sleep apnea Sleep apnea is a serious disorder in which breathing repeatedly stops long enough to disrupt sleep and often temporarily decrease the amount of oxygen and increase the amount of carbon dioxide... read more 
.
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 often have delays in developing motor and language skills, but this is variable. Behavior suggestive of attention-deficit/hyperactivity disorder Attention-Deficit/Hyperactivity Disorder (ADHD) Attention-deficit/hyperactivity disorder (ADHD) is poor or short attention span and/or excessive activity and impulsiveness inappropriate for the child’s age that interferes with functioning... read more is often seen in childhood. Children with Down syndrome are at greater risk of autistic behavior, especially those with severe intellectual disability Intellectual Disability 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 . There is an increased risk of depression among adults Depression Depression is a feeling of sadness and/or a decreased interest or pleasure in activities that becomes a disorder when it is intense enough to interfere with functioning. It may follow a recent... read more and depression among children Depression and Mood Dysregulation Disorder in Children and Adolescents Depression includes a feeling of sadness (or, in children and adolescents, irritability), and/or loss of interest in activities. In major depression, these symptoms last 2 weeks or more and... read more . Early intervention with educational and other services improves the functioning of young children with Down syndrome.
Diagnosis of Down Syndrome
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
(See also Next-generation sequencing technologies Next-generation sequencing technologies Genetic diagnostic technologies are scientific methods that are used to understand and evaluate an organism's genes. (See also Genes and Chromosomes.) Genes are segments of deoxyribonucleic... read more .)
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. If doctors suspect Down syndrome based on these screening tests, they often confirm the diagnosis using chorionic villus sampling Chorionic Villus Sampling 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 , amniocentesis Amniocentesis 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 both ( see Testing for chromosome and gene abnormalities Testing for chromosome and gene abnormalities Chromosomes are structures within cells that contain a person's genes. A gene is a segment of deoxyribonucleic acid ( DNA) and contains the code for a specific protein that functions in one... read more ).
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 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 Sleep Apnea Sleep apnea is a serious disorder in which breathing repeatedly stops long enough to disrupt sleep and often temporarily decrease the amount of oxygen and increase the amount of carbon dioxide... read more . 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.
Prognosis for Down Syndrome
Down syndrome has a better prognosis in comparison to most other disorders caused by an extra chromosome, such as trisomy 18 Trisomy 18 Trisomy 18 is a chromosomal disorder caused by an extra chromosome 18 that results in intellectual disability and physical abnormalities. Trisomy 18 caused by an extra chromosome 18. Infants... read more 
or trisomy 13 Trisomy 13 Trisomy 13 is a chromosomal disorder caused by an extra chromosome 13 that results in severe intellectual disability and physical abnormalities. Trisomy 13 is caused by an extra chromosome 13... read more 
.
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 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 people with Down syndrome.
Recent findings indicate that Black people with Down syndrome have a substantially shorter life span than White people. This finding may be the result of poor access to medical, educational, and other support services.
Treatment of Down Syndrome
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 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 ).
More Information
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