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Down's syndrome

People with Down's syndrome have an extra chromosome in some or all of their body's cells, resulting in certain physical characteristics and some level of learning difficulty. Down's syndrome affects one in every 1000 babies in the UK.

About Down's syndrome

Down's syndrome is caused by an extra chromosome, one of the tiny bundles of DNA found in all of our cells.

Normally, our cells contain 46 chromosomes: 23 inherited from each parent. In Down's syndrome, an extra copy of chromosome 21 is included when the sperm and the egg meet to form the embryo. This means there are 47 chromosomes in the baby's cells. Down's syndrome is also known as trisomy 21.

On rare occasions an extra fragment of chromosome 21 becomes incorporated into another chromosome: this is called translocation and it happens in about 6 percent of people with Down's syndrome. Occasionally, only some cells have an extra chromosome 21 while others don't. This is called mosaicism, and about 2 percent of people with Down's syndrome have this form of the condition.


The extra chromosome 21 causes characteristic physical features in people with Down's syndrome. These usually include some, but not always all, of the following:

  • low-set eyes that slope upwards, with vertical skin folds (epicanthic folds) between the upper eyelids and the inner corner of the eye
  • small mouth, which means the tongue may seem big and may stick out
  • flattening at the back of the head
  • a flattened nose bridge
  • broad hands with a single crease
  • floppiness due to loose muscle tone
  • small, low-set ears
  • low birth weight and short stature

Many of these physical features are found in the general population: having some of these characteristics does not necessarily mean that a person has Down's syndrome.


People with Down's syndrome are more likely than most people to get a range of medical conditions. They are summarised below.

  • Heart problems, which may need surgery, affect up to half of people with Down's syndrome.
  • Eye problems, such as short- or long-sightedness or cataracts affect half of children with Down's syndrome by the age of four.
  • Hearing problems, ranging from mild to complete deafness, affect over half of people with Down's syndrome.
  • Thyroid problems, including low or high levels of the thyroid hormones, affect one in 10 people with Down's syndrome.
  • Poor immunity is common, so people with Down's syndrome are prone to chest infections, coughs and colds.
  • Problems with the digestive system may cause diarrhoea or constipation; babies may have feeding problems and may not gain weight normally.
  • Dementia affects people with Down's syndrome at an earlier age: four out of ten 50-59 year-olds with Down's syndrome have dementia.

Many of these problems can be treated, but they need to be diagnosed by a doctor. It's important for people with Down's syndrome to have health checks.


All people with Down's syndrome have some level of learning disability. Children usually learn to walk, talk, read and write, but more slowly than other children of their age. People with Down's syndrome learn to do things throughout their lives at different rates, just as the general population do.


Nobody knows why some women have babies with Down's syndrome and others do not. However, the chance of having a baby with Down's syndrome increases with age. If you are 20, the chance of having a baby with Down's syndrome is one in 1440; at 35 it's one in 338; and at 45 it's one in 32. Most babies with Down's syndrome are born to women under 35, since these women account for the majority of the childbearing population.

The chance of you having a baby with Down's syndrome has nothing to do with where you live, your social class or your race. You can't do anything before or during pregnancy to change the chance of your baby having Down's syndrome.


Babies with Down's syndrome are usually diagnosed in the first few days after birth. Doctors and midwifes are trained to identify the physical characteristics associated with the condition. Some babies have almost no physical signs while others have all of them.

Living with Down's syndrome

People with Down's syndrome have special medical and social needs, but they can live full lives, taking part in further education, having jobs and relationships, and living independently.

Medical and social support

A team of professionals will help support people with Down's syndrome, and their families. This team may include your GP, paediatricians, midwives, health visitors, occupational and speech therapists and physiotherapists.

Specialist doctors monitor all babies with Down's syndrome for health problems, and children with Down's syndrome have regular growth, hearing, sight and thyroid checks. Adults should also have regular sight, hearing and thyroid function tests.

Occupational therapists and dieticians can help parents with issues such as nutrition and educational support for their children. Most children with Down's syndrome go to mainstream schools, but there are schools for children with special needs.

For more advice from other organisations, such as the Down's Syndrome Association, who provide support for people with Down's syndrome and their families, see Further information.

Further information

Down's Syndrome Association


  • Down's syndrome. Learning about intellectual disabilities and health. St Georges University of London. www.intellectualdisability.info, accessed 4 April 2007
  • Simon C, Everitt H, Kendrick T. Oxford Handbook of General Practice. 2nd edition. Oxford University Press: 2006: 844
  • Down's syndrome, clinical features. GP Notebook. www.gpnotebook.co.uk, accessed 26 March 2007
  • Medical library. Down's syndrome medical interest group. www.dsmig.org.uk, accessed 3 April 2007
  • Pueschel SM. 1990. Clinical aspects of Down's Syndrome from Infancy to Adulthood. American Journal of Medical Genetics, Supplement 7:52-56
  • Access to Education. Down's syndrome association. www.downs-syndrome.org.uk, accessed 4 April 2007

Related topics

  • Looking after your heart
  • Short-sightedness
  • Hearing loss
  • Overactive thyroid
  • Underactive thyroid
  • Dementia