Down syndrome or Down's syndrome is a genetic disorder that affects hundreds of babies worldwide. The condition is caused by the presence of an additional copy of chromosome 21 in a person’s cells.

Humans usually have 46 chromosomes in every cell, with 23 inherited from each parent. Due to the extra copy of chromosome 21, people with Down syndrome have 47 chromosomes in their cells. This additional DNA causes the physical characteristics and developmental problems associated with the syndrome.

The extra copy of chromosome 21 is acquired by chance and although Down syndrome is more common among babies born to mothers of an older age, mothers of any age may have a baby with the condition.

Worldwide, the incidence of Down syndrome is estimated to be about one in every 1000 births. In the United States, it is estimated that about 6000 babies are born each year with Down syndrome, which means around one in every 700 babies has the condition.

There are three different types of Down syndrome but all give rise to similar physical and behavioural features. In about 95% of cases, babies are born with a whole and separate extra copy of chromosome 21.

In around 3% of cases, a part or a whole extra copy of the chromosome occurs but is attached to another chromosome rather than existing as a separate copy. And in around 2% of cases, “mosaic Down syndrome” occurs, which describes a condition where a proportion but not all of a person’s cells have an extra copy of chromosome 21.

Children with the mosaic form of the condition may have less of the characteristic features of Down syndrome due to a certain proportion of their cells having a normal number of chromosomal copies.

Down syndrome can affect individuals of any races or ethnicity and is the most common genetic cause of learning disability. Research has shown a link between advancing maternal age and the risk of Down syndrome developing in babies.

Women who are aged 35 years or older when they become pregnant have a greater likelihood of giving birth to a baby with Down syndrome than women who are younger than 35 when they become pregnant. However, most babies with Down syndrome are born to women under the age of 35, due to the higher rate of births among women below this age.

Studies have also shown that older age of the father also increases the risk of Down syndrome in offspring when the mother is aged over 35 (but not in women aged under 35) when she becomes pregnant.