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What is Down Syndrome?

Last updated on 28th April 2023

Down syndrome is the most common genital chromosomal condition, and the most prominent cause of learning difficulties and disabilities in the UK. Down syndrome occurs in 1 in every 700 births in the UK.

What is Down syndrome?

Down syndrome is a genetic disorder where an individual is born with an extra chromosome. Someone with the condition is usually easily identifiable visibly, due to the very distinctive features of the syndrome. Down syndrome is sometimes referred to as trisomy-21, T21 or Down’s syndrome. It is also sometimes referred to as Down’s.

The condition was named after physician John Langdon Down, who described symptoms of the syndrome in 1866, but referred to it as Mongolism instead. French physician Jérome Lejeune also made significant strides in understanding Down syndrome in 1959, when he observed the additional cells in people with Down syndrome.

How does Down syndrome occur?


Down syndrome is caused by the foetus’ cells dividing abnormally, which results in an extra chromosome, chromosome 21. The abnormal cell division is called ‘nondisjunction’. Each human should have 23 pairs of chromosomes, which are responsible for fetal development and how a child continues to develop once they are born. However, someone with Down syndrome has 47 chromosomes, instead of 46. When chromosome 21 is copied, either fully or partially, it affects the development of the brain and the body.

The development of Down syndrome is due to an abnormality in the sperm or the egg either at or after conception, where either one does not separate their pair of 21st chromosomes properly. This accounts for the majority of cases of Down syndrome.


Another way that Down syndrome is caused is by a process called Mosaicism, where an extra chromosome 21 blends with other chromosomes. However, this form of Down syndrome is uncommon. Studies have shown that people who have this type of Down syndrome tend to display fewer symptoms and characteristics of the condition, though this can still vary from person to person.


Translocation refers to another way that people can develop Down syndrome. In translocation, chromosome 21 copies either in full or partially to chromosome 14. It is still unknown what causes the cells to divide abnormally, though much evidence has indicated that maternal age plays a factor, with children born to older women more likely to develop Down syndrome.

Down Syndrome Condition

Is Down syndrome inherited?

Down syndrome is a genetic condition, though only a small number of cases (1%) of the condition are inherited through parents. Thus, contrary to popular belief, heredity does not play a significant part in Down syndrome. It is also very unlikely that a woman who has given birth to one baby with Down syndrome will mother another child with the condition, though it can happen. Translocation is genetically passed down to the foetus, although this type of Down syndrome is rare.

What are the signs and symptoms of Down syndrome?

Symptoms of Down syndrome are different in everyone, but it is characterised by some common visible features, including:

  • A flattened face.
  • Slanted eyelids that create an almond shape.
  • Smaller than average head size.
  • Short neck with additional skin at the back of the neck.
  • Flat nasal bridge.
  • Protruding tongue.
  • Below average height (only usually noticeable as they leave the infant stage).
  • Short fingers.
  • Small hands.
  • Small feet.
  • White spot on the iris of the eye.
  • One crease in the palm of the hand.
  • Small or misshapen ears.
  • Weak muscle tone.

Other symptoms of Down syndrome include intellectual disabilities. Most children who have Down syndrome will have an intellectual disability, which will usually affect them mildly to moderately. Children with Down syndrome tend to learn considerably more slowly than children without the condition.

Learning disabilities and difficulties impact the following areas:

  • Motor skills – Children with Down syndrome display a slower rate of motor skill development than other children, which further impacts their cognitive development, due to limited interaction.
  • Speech – Down syndrome affects oral motor skills, and the ability to learn and retain language. This is commonplace for people with the condition, who will struggle to produce accuracy in their sentences and clear speech.
  • Memory – Children with Down syndrome often struggle to retain information, which affects their ability to produce speech. This is helped by the use of visual aids.
  • Numbers – Many children with Down syndrome have difficulties in using and understanding numbers, perhaps due to their memory retention and poor processing.

There are also a number of potential health conditions that people with Down syndrome may develop, and which may affect them more significantly as they get older. Some complications may be picked up during pregnancy, whilst the other problems develop over time.

These may include:

  • The majority of people with Down syndrome tend to have heart problems. The most common heart defect is an atrioventricular septal defect (AVSD), which is when there are holes between the right and left sides of the heart, and the blood does not flow properly between the two sides.
  • Leukaemia – According to Leukaemia Care, children and teenagers with Down syndrome are up to 30 times more likely to develop leukaemia.
  • Dementia – As people with Down syndrome reach the age of 40, their risk of developing dementia increases. People with Down syndrome have protein deposits which affect their brain cell function. This also increases the risk of Alzheimer’s.
  • Immune conditions – People with Down syndrome have a higher risk of autoimmune disorders and susceptibility to respiratory infections. They are also more at risk of developing pneumonia, sepsis and different forms of cancer.
  • Obesity – People with Down syndrome are at a higher risk of being overweight due to a lower metabolic rate and decreased thyroid function.
  • Spinal issues – People with Down syndrome may experience misalignment in the vertebrae of their neck, which can lead to severe neck injury.
  • Sleep apnoea – People with Down syndrome may experience airway obstruction which can lead to sleep apnoea.
  • Gastrointestinal conditions – People with Down syndrome may have a wide range of issues with gastrointestinal function, including reflux, diarrhoea, constipation, and some may have coeliac disease.

For people with Down syndrome, lifespan is around 60 years, and this is projected to continue to rise.

Child With Down Syndrome

What are the risk factors for Down syndrome?

As mentioned, older maternal ages are associated with an increased risk of Down syndrome in the child. This is because as a woman gets older, so do her eggs, and older eggs have a greater chance of irregular chromosomal division. Women over the age of 35 have a greater risk of their child developing Down syndrome.

Additionally, if the mother or father has the translocation type of Down syndrome, there is a small risk that this can be passed on to one or more of their children.

Currently, there are no known environmental factors that can cause Down syndrome.

How is Down syndrome diagnosed?

Down syndrome is usually diagnosed using screening tests during pregnancy. This is carried out during the scan between 11 to 14 weeks. The initial test that is carried out is a combined test, which screens for Down syndrome, Edwards’ syndrome and Patau’s syndrome, using ultrasound and a blood test.

This test is optional, but if it is taken, the fluid at the rear of the foetus’ neck is measured, depending on the position of the baby, and results are assessed with the blood test results and the mother’s age, to determine the risk of Down syndrome. If it is believed that there is a significant risk of Down syndrome, further tests will be performed to see if the baby has Down syndrome. If the baby is not in an optimal position to measure the fluid at the back of the neck, the quadruple test will be taken between 14 to 20 weeks of pregnancy, but this is less accurate.

The initial testing does not harm the mother or the baby. However, if the test indicates that the baby may have Down syndrome, further diagnostic tests to determine whether the baby definitely has it can lead to miscarriage in some cases. For this reason, some people choose not to take the test, and instead just have a 12-week scan. It is also common to have a second screening if the initial test indicates a concern, called a non-invasive prenatal test (NIPT). This is another blood test which can help to give a more accurate picture without the risk of miscarriage. In the UK, 720,000 mothers will be offered a test, and more than 95% of tests in the UK indicate a very low chance of Down syndrome. The Down Syndrome Medical Interest Group found that in 2018, 85.2% of pregnancies that could potentially result in a Down syndrome child were terminated in the UK.

How is Down syndrome treated?

There is no cure for Down syndrome, and instead, treatments are used to help reduce the impact of some of the complications involved with the condition. With early treatment, many people with Down syndrome can live as comfortably as possible.

When a baby is identified as having Down syndrome, early intervention programmes are set up for after their birth, which includes speech and language therapy and other techniques that can help with learning difficulties. Research has shown that the earlier the interventions occur, the more effective the outcomes. It can usually begin just after birth, up to the age of 3 years old, after which, the child’s school and local authorities will be responsible for a plan of care. The interventions will include many different healthcare practitioners, such as paediatricians, cardiologists, physiotherapists and speech therapists.

Playing With Toys

Living with Down syndrome

Many children with Down syndrome live a full, happy life. The average life expectancy for someone with Down syndrome has increased rapidly over the last six decades, going from just 25 to 60 years old, with many people living until much older. As social attitudes change and awareness increases, people with Down syndrome are experiencing a much better quality of life. Many adults with Down syndrome live independently, have a job, have relationships and play an active role in society.

However, people with Down syndrome face many challenges. As they are at a higher risk of developing significant health conditions, this can severely impact their ability to function. Intellectual disabilities and learning difficulties are common. Additionally, due to their facial features and demeanour, it is sometimes assumed that people with Down syndrome are always happy. This is not the case. In fact, people with Down syndrome are more likely to be depressed or anxious.

People with Down syndrome should be referred to as individuals first, rather than as a ‘Down’s child’. Some people sometimes describe someone with Down syndrome as ‘he/she’s Down’s’. This can lead people with the condition to feel unseen and defined by their condition. Additionally, people with Down syndrome have many unique qualities and strengths in that they are often very warm and loving, with strong visual awareness. Whilst children with the condition may develop more slowly, they can be particularly skilled at interacting with other children, an advantage that other children may not have.

Who can support people with Down syndrome?

  • Antenatal Results and Choices is a charity dedicated to helping expecting parents through the process of antenatal screening and its consequences. They provide objective information about screening results, so that parents can make informed decisions. They offer support for parents who decide to terminate their pregnancy, and those who decide to continue.
  • The Down’s Syndrome Association is an organisation dedicated to improving the quality of life for people with Down syndrome. They offer training sessions for inclusive participation, and help to support education, speech, employment and conditions related to Down syndrome.
  • The Down’s Syndrome Research Foundation UK provides funding for medical research on Down syndrome, and aims to improve awareness and understanding around the condition.
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About the author

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Rose Winter

Rose is a qualified teacher with six years of experience teaching in secondary schools and sixth forms across London. Before this, she worked as a communications officer in the Cabinet Office. Outside of work, Rose can be found researching topics of interest and spending time abroad.

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