Causes - Down's syndrome

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

Chromosomes explained

Our bodies are made up of cells that contain genes. Genes are grouped in thread-like structures called chromosomes.

These contain detailed genetic instructions for lots of different things, including:

  • how a baby's cells develop
  • their gender
  • their eye colour

Usually, cells contain 46 chromosomes – 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 instead, as there's an extra copy of chromosome 21. This extra gene causes the characteristics of Down's syndrome.

In most cases, Down's syndrome isn't inherited, it's just the result of a one-off genetic change in the sperm or egg.

What are the chances of having a baby with Down's syndrome?

With every pregnancy, there's a small chance of having a baby who has Down's syndrome.

Some people are more likely to have a child with Down's than others.

The main thing that increases the chance of having a baby with Down's syndrome is the mother's age.

For example, a woman who is:

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

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

Your chance of having a child with Down's syndrome is also increased if you previously had a child with Down's. For most people, this chance is still small (around 1 in 100).

There's around a 1 in 2 chance of a child having Down's syndrome if one of his or her parents has the condition.

Page last reviewed: 30/04/2017
Next review due: 30/04/2020