• Also Known As:
  • Trisomy 21
  • DS
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What is Down syndrome?

Down syndrome (trisomy 21 or DS) is a group of signs, symptoms, and health problems that arise from an error in cell division that results in an extra chromosome 21. This may occur before or shortly after conception and has a widespread effect on the physical and intellectual development of the affected person. DS is one of the most common and well known chromosome disorders and the most common genetic cause of learning disabilities.

Chromosomes hold the body’s genetic blueprint. Most cells in the body contain 22 pairs of chromosomes and a 23rd set of either XX (in females) or XY (in males) for a total of 46 chromosomes. The 46 chromosomes come together when the woman’s egg, containing a single set of 23 chromosomes, is fertilized by a single sperm, containing a single set of 23 chromosomes. The fertilized egg will then divide repeatedly as it grows and develops into a fetus.

  • In most cases of Down syndrome, the two copies of chromosome 21 in a parent cell do not separate properly when an egg or sperm cell is made, and the extra copy is included in the new egg or sperm cell. This is called “nondisjunction.” This extra copy becomes part of the fertilized egg and is replicated in all of the embryo’s cells. This form of Down syndrome is called trisomy 21 (because the embryo got two copies of chromosome 21 from one parent, and 1 copy from the other), and it accounts for about 95% of DS cases.
  • Less commonly, an error may occur after conception, in the developing embryo. As the fetus grows, some cells have 47 chromosomes, while others have 46. This form of Down syndrome is called mosaic trisomy 21 or mosaicism. This is the least common form of down syndrome, accounting for just 1% of cases.
  • A Third rare form of Down syndrome is called translocation trisomy 21. A piece of chromosome 21 adheres to another chromosome before or at conception. Even though the fetus has 46 chromosomes, there is still an extra portion of chromosome 21 in the cells. This accounts for the remaining 4% of Down syndrome cases.

All individuals with additional chromosome 21 genetic material, regardless of the cause, will develop some of the features of Down syndrome. Which specific features and health problems are present and how severe they are will vary between individuals.

In the United States, about 6,000 babies each year, or 1 in 700, are born with Down syndrome. Maternal age is the only factor that’s been linked to an increased risk for having a baby with Down syndrome. The risk of having an affected baby increases significantly as a woman ages, according to the American College of Obstetricians and Gynecologists.

The following table summarizes the risk by age:

Mother’s Age Risk of Having a Baby with Down Syndrome
20 1 in 1,480
30 1 in 940
40 1 in 85
45 1 in 35

However, younger women have the most babies, so around 80% of babies with Down syndrome are born to women under 35.

There’s no evidence to show that Down syndrome can be caused by environmental factors or any activities before or during pregnancy. Although Down syndrome is a genetic condition, only translocation trisomy 21 has the potential to be hereditary.


About Down Syndrome

Signs and Symptoms

There are many characteristic signs and symptoms associated with Down syndrome. Not every child will have every one, and the degree to which they are affected may vary greatly. Some physical signs and symptoms include:

  • A small head with small, low-set ears
  • Short neck with excess skin on the back
  • Up-slanting eyes, often with a skin fold at the inner corner, a broad flat face, and a short nose
  • A small mouth and protruding tongue
  • Brushfield spots (white spots on the pigmented part of the eye)
  • Short, small but broad hands and feet and a single crease across the palm
  • Short fingers and an abnormal bone in the 5th (pinky) finger
  • Poor muscle tone
  • Hyperflexible joints
  • Atlantoaxial instability (a malformation of the top of the spine)
  • Delayed physical development (increased time to reach childhood physical milestones)

Problems with thinking and learning (cognitive impairment) and intellectual disability range from mild to moderate and may include:

  • A short attention span
  • Hyperactivity
  • Poor judgment
  • Disruptive or impulsive behavior
  • Delays in language and speech development
  • Anxiety
  • Sleep difficulties

Associated Conditions

People with Down syndrome are at an increased risk of certain conditions compared to the general population. Some of these conditions may be present at birth, some may arise during childhood, others during adulthood, and others may never develop. Healthcare practitioners and family members must be aware of these potential issues because individuals with DS may or may not be able to clearly communicate their symptoms or they may express them in unexpected ways.

Examples of conditions associated with Down syndrome include:

  • Attention deficit hyperactivity disorder (ADHD)
  • Autism spectrum disorder
  • Dental disease
  • Digestive tract issues (e.g., food sensitivities, constipation, obstructions, increased celiac disease risk, with about 16% of people with Down syndrome affected)
  • Hearing loss
  • Heart defects and heart disease (close to 50%)
  • More frequent or chronic respiratory infections, ear infections, colds, bronchitis, tonsillitis, and pneumonia
  • Increased risk of blood disorders (e.g., myelodysplastic syndrome, leukemia)
  • Premature aging, loss of cognitive abilities, and Alzheimer disease (about 30% by age 50 and 50% by age 60)
  • Seizure disorders (epilepsy)
  • Sleep apnea (50-100%)
  • Spinal cord compression
  • Thyroid disease (about 10% in children and 13% to 50% in adults with DS)
  • Visual problems, including cataracts (over 50%)


Screening and Diagnostic Tests

Screening and diagnostic tests may be done during a woman’s pregnancy. Screening tests are not diagnostic. If a screening test indicates an increased likelihood of the fetus having Down syndrome, then a diagnostic test (e.g., amniocentesis) is recommended. The American College of Obstetricians and Gynecologists (ACOG) has recommended that all pregnant women be offered DS screening tests.

Prenatal screening tests

  • Non-invasive prenatal screening (NIPS, NIPT) – analyzes cell-free fetal DNA released from the placenta and circulating in the mother’s blood, The test is recommended as early as 10 weeks of pregnancy, especially for women who are considered at a higher risk for having a baby with Down syndrome.
  • First trimester screen – nuchal translucency (non-laboratory test, see below), pregnancy-associated plasma protein A (PAPP-A), and free beta or total hCG (human chorionic gonadotropin), usually performed between 11 and 14 weeks of pregnancy
  • Maternal serum screen (second trimester screen or triple/quad screen) – alpha feto-protein (AFP), chorionic gonadotropin (hCG), and unconjugated estriol (uE3); quad screen adds inhibin A test; performed at 15 to 20 weeks of pregnancy
  • Sequential and integrated maternal serum screening – combined first and second trimester screening test for open neural tube defects (ONTD), DS, and trisomy 18 (T18). Tests are performed at the times noted in the two bullets above. (See the article on the Maternal Serum Screen for more on this approach.)

Prenatal diagnosis
Prenatal diagnostic tests may be performed when screening tests show an increased risk. A healthcare practitioner removes samples of the fluid or tissues surrounding the baby and they are tested for an additional copy or portion of chromosome 21 using chromosome analysis or chromosomal microarray. A risk of infection and miscarriage (1%) is associated with these sampling techniques.

Diagnosis after birth
A diagnosis after birth is usually based initially on physical signs and characteristics. However, these signs and symptoms may also be seen in babies without Down syndrome. Diagnostic testing performed after birth involves taking a sample of blood from the babies and evaluating their chromosomes:

  • Chromosome analysis (karyotype) – cells are cultured from a blood sample and chromosomes are evaluated for an extra copy of chromosome 21; the presence and type of Down syndrome can be determined from this test.

Prenatal screening and diagnosis are performed to detect the condition in the fetus and to help pregnant women, their families, and their healthcare practitioners make decisions about managing their pregnancies.

Tests for associated conditions
Some tests may be used to detect and monitor for medical conditions often seen in those with Down syndrome. Some of the medical issues may be present at birth and others may develop later in life. A variety of laboratory tests may be used to detect these conditions as they arise. Some examples include:

  • TSH and/or T4 – to detect thyroid disease, especially hypothyroidism. This condition may develop at any age for a person with Down syndrome, so screening is usually done as a newborn and then at regular intervals throughout the person’s life.
  • Complete blood count (CBC) – to detect infections, anemia or sometimes other blood disorders (e.g., MDS, leukemia)
  • Celiac disease antibody tests – these tests may be recommended for children with Down syndrome as young as 2 years old or younger if they have signs and symptoms of the condition.

Non-Laboratory Tests


  • Nuchal translucency – an ultrasound measurement of the fluid between the fetal spine and the skin at the back of the neck; not diagnostic, but in a fetus with Down syndrome, there may be an increased amount of fluid. This test requires a person with specialized training to perform and interpret. It may be performed as part of the first trimester pregnancy screen.
  • Second trimester high-resolution ultrasound – can help monitor fetal development and detect malformations, such as heart and digestive tract defects

At or soon after birth

  • Echocardiogram and chest x-rays  – to help detect heart defects
  • Ultrasound and/or MRI (magnetic resonance imaging) – to evaluate any suspected congenital conditions, such as heart defects and obstructions in the digestive tract
  • Hearing evaluation


Currently there is no way to prevent or cure Down syndrome. Treatment is directed to the signs and symptoms of each individual, as needed. Approximately 25% to 30% of babies with Down syndrome die within the first year of life, but early diagnosis allows the family and healthcare practitioner to work together to monitor the baby and to prepare for medical issues that may require attention shortly after birth. Medical treatments may include surgical interventions, such as repairing heart defects and removing obstructions in the digestive tract, and starting medications for conditions such as thyroid disease.

In individuals with Down syndrome, careful monitoring, prompt attention to acute and chronic conditions that arise, and early intervention to maximize the potential of the individual are important. The symptoms, signs, medical issues, and abilities of people with Down syndrome will vary widely. Families should work closely with their healthcare practitioners and other specialists to develop plans that meet the unique needs of those affected.

There are national, state, and local early intervention programs and resources that can help children with Down syndrome develop their physical, communication, and cognitive skills. See the links in Related Content for more details on treatments.

View Sources

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