Down's syndrome Dr Trisha Macnair Dr Rob Hicks Down's syndrome is a genetic abnormality that causes physical and intellectual impairments.

In this article

What is it? Different types

What are the symptoms? What's the treatment?

Who's affected? Antenatal screening

Down's syndrome (also called Down syndrome) is a genetic condition known as trisomy, where a person inherits an extra copy of one chromosome. People with the syndrome have three copies of chromosome 21 rather than two.

This additional genetic material changes the finely tuned balance of the body and results in characteristic physical and intellectual features.

It's not known why this abnormality occurs, but it affects around one in 1,000 babies born in the UK - about 750 babies a year - and is the most common cause of learning disability.

There are three types of Down's syndrome:

Regular trisomy 21 - all the cells have an extra chromosome 21. Around 94 per cent of people with Down's syndrome have this type.

Translocation - the extra chromosome 21 material is attached to another chromosome and one of the parents may carry the translocated chromosome without any signs of the condition themselves. This accounts for around 4 per cent of cases.

Mosaic - only some of the cells have an extra chromosome 21. Around two per cent of people with Down's have this type, which tends to result in milder features.

People with Down's syndrome tend to look different – they typically have a flat facial profile and eyes that slant upwards. Other facial features include smaller ears, a flat back of the head and protruding tongue.

People with the syndrome also tend to be shorter than average with poor muscle tone and have short, broad hands with a single crease across the palm.

Almost half of people affected have heart defects, some of which can be treated, some of which can't. Many also have gut problems, which can make eating difficult and increases the risk of problems such as constipation and disorders of the thyroid gland.

Other physical problems include cataracts, hearing and sight problems, and a susceptibility to infections. Later in life there's also an increased risk of leukaemia and Alzheimer’s dementia.

People with Down's syndrome have varying degrees of learning disability, which may range from moderate to severe. Autistic spectrum disorders are also more common.

There's no cure, but treatment of any accompanying health problems and support for learning difficulties allows many people with the syndrome to lead relatively normal and semi-independent lives. Others, however, need full-time care. Many people with the condition live well into adulthood, with an average life expectancy of around 60 years.

Physiotherapy, speech therapy and special educational programmes have an important role to play, while specific medical conditions associated with the syndrome are treated as appropriate.

Maternal age

The likelihood of having a baby born with Down's syndrome increases with the mother's age:

20 years - 1 in 1,500
25 years - 1 in 1,300
30 years - 1 in 900
35 years - 1 in 350
40 years - 1 in 100
45 years - 1 in 30
Please note these are approximate figures and that maternal age is taken by convention to refer to age at estimated or actual delivery of the baby. Also note that the risk of conceiving a baby with Down's syndrome is higher than the above figures but many are lost early in the pregnancy through spontaneous miscarriage.

The chance of having a baby with Down's syndrome increases with the age of the mother, especially over the age of 35. But as the majority of women have their children when they're younger, most Down's babies are born to younger parents.

Having a child with Down's syndrome is more likely if a woman already has a child with the syndrome, or if a close family member has had an affected child. But most cases are sporadic (the parents aren't affected) and the risk of recurrence in further pregnancies is very small.

Down's syndrome affects people from all ethnic groups and cultures.

It's not possible to prevent the genetic abnormality that causes Down's syndrome, but in recent years it has become possible to identify more accurately during pregnancy those babies most at risk.

The most effective and safe method of antenatal screening for Down's syndrome is known as the 'combined' or 'triple' test, which is carried out after about week ten or 11 of the pregnancy and before week 14. The test combines information from an ultrasound scan of the baby (this test, which measures the spinal cord at the back of the baby’s neck, is called a nuchal translucency measurement) and results of blood tests which measure certain chemicals (free beta-hCG and PAPP-A) in the mother's blood.

This test is routinely offered in most antenatal clinics (in some situations a more limited test may be offered). The results are then combined with the woman's age at her expected date of delivery in order to give an estimated level of risk of Down's syndrome.

Women who need screening later in their pregnancy may be offered the “quadruple” test which involves a blood test for 4 different chemicals (alpha-fetoprotein, unconjugated oestriol, free beta-hCG and inhibin-A) combined with maternal age.

It's important for parents to realise that screening only identifies those women who are at an increased risk of having a baby with Down's and estimates the size of that risk - it doesn't actually tell them for certain whether or not their baby has Down's.

Those women who are found to be at a higher risk of having a baby with Down's syndrome are offered the opportunity of further tests to establish whether or not their child is affected. The tests offered may be amniocentesis or chorionic villus sampling. These provide a definitive answer, but are invasive and can trigger a miscarriage.

Screening for Down's syndrome isn't compulsory. Women are offered screening if they wish to know the likelihood of having a baby with Down's syndrome. If they are at high risk, they can then decide whether to have a definitive test.

If this test confirms Down's syndrome, women can then make an informed decision about continuing with their pregnancy or not.

Advice and support

Down's Syndrome Association
Tel: 0845 230 0372
Website: www.downs-syndrome.org.uk

Down's Syndrome Scotland
Tel: 0131 313 4225
Website: www.dsscotland.org.uk

Down Syndrome Educational Trust
Tel: 023 9285 5330
Website: www.downsed.org

This article was last medically reviewed by Dr Trisha Macnair in November 2008.