Down's syndrome, (Trisomy 21), is the most common cause of severe mental handicap. About 1000 new cases occur in the UK each year. All children with Down's syndrome have a degree of learning disability and although they have special educational needs, many attend mainstream schools.

The ability of adults with Down's syndrome varies considerably. This is reflected in the degree of independence and level of employment.

Certain medical conditions are more common in people with Down's syndrome; these include dry skin, slow feeding, poor tongue control and a tendency to develop chest and sinus infections. Some 40% of babies with Down's syndrome are born with a heart defect, ranging from a slight murmur to a severe abnormality requiring surgery. Hearing, vision and thyroid problems may also occur. However, many people with Down's syndrome enjoy a healthy life, and a life span of 40 - 60 years is not uncommon.

Down's syndrome is the result of a chromosomal abnormality; it can be inherited, though this rarely happens, and most cases occur in couples with no family history. Every human cell contains chromosomes, which incorporate the genes that influence our individual characteristics. In a normal human cell there are 46 chromosomes (23 pairs), but in individuals with Down's syndrome there is an extra chromosome 21 so each cell has 47 chromosomes. Most cases are known to result from an error in cell division during the early stages of egg production. There are other serious chromosomal abnormalities, such as Edwards' syndrome (Trisomy 18), but Down's syndrome is by far the most common.

The chance of having an affected baby increases with the mother's age; the age-related risk is about 1 in 900 for women aged 30 and 1 in 30 for women aged 45. However, most babies with Down's syndrome are born to younger women - about half of these are age 30 or under - since most pregnancies occur in this age group.