Down's syndrome is a genetic condition that occurs as a result of an extra chromosome (chromosome 21).

Chromosomes

The human body is made up of cells that contain genes. Genes are grouped in thread-like structures called chromosomes.

Chromosomes are blocks of DNA and contain detailed genetic instructions responsible for a wide range of factors, including:

• how the body's cells develop
• the colour of the eyes
• the sex of a baby

Usually, cells contain 46 chromosomes. A child inherits 23 from the mother and 23 from the father.

In people with Down's syndrome, all or some of the cells in their bodies contain 47 chromosomes, as there is an extra copy of chromosome 21. The additional genetic material causes physical and developmental characteristics associated with Down's syndrome.

Types of Down's syndrome

There are three forms of Down's syndrome, although the effects of each type are usually very similar.

• Trisomy 21 is the most common type, affecting around 94% of people with Down's syndrome. It's where every cell in the body has an extra copy of chromosome 21.
• Translocation is where an extra piece of chromosome 21 attaches itself to another chromosome. It affects around 4% of people with Down's syndrome. In this situation, there are only 46 chromosomes in each cell but one of them has extra material attached, so the total amount of genetic material in the cell is similar to that seen in a trisomy.
• Mosaicism is the least common type, affecting around 2% of people with Down's syndrome. Only some of the cells have an extra copy of chromosome 21, which can mean people with mosaic Down's syndrome experience less delay with some aspects of their development.

In the vast majority of cases, Down's syndrome isn't inherited, and is simply the result of a one-off genetic mistake in the sperm or egg.

However, in some cases caused by the translocation of chromosome 21, the genetic mistake is passed on from one of the child's parents, even if they have no signs of Down's syndrome themselves.

What are the chances of having an affected pregnancy?

With every pregnancy, there is a small chance of having a child with Down's syndrome.

There's currently no clear evidence to suggest that anything done before or during pregnancy increases or decreases this risk, although some people are more likely to have a child with Down's syndrome than others.

The main factor that increases the chance of having a baby with Down's syndrome is the age of the woman when she becomes pregnant. For example, a woman who is:

• 20 years of age has a risk of one in 1,500
• 30 years of age has a risk of one in 800
• 35 years of age has a risk of one in 270
• 40 years of age has a risk of one in 100
• 45 years of age has a risk of one in 50 or greater

However, babies with Down's syndrome are born to mothers of all ages.

The chance of having a child with Down's syndrome is also increased if you have previously had an affected pregnancy. For most people this risk is still small (around one in 100), although this may be as high as one in 10 if you or your partner have the altered genes that can lead to the translocation of chromosome 21.

There is around a one in two chance of a child having Down's syndrome if one of his or her parents has the condition.