What causes Down syndrome?

Down syndrome is a genetic condition caused by the presence of an extra chromosome 21. A baby born with Down syndrome has three copies of chromosome 21 instead of the usual two.

As yet it is not known what causes the presence of an extra chromosome 21.  It can come from either the mother or the father.  There is no way of predicting whether a person is more or less likely to make an egg or sperm with 24 chromosomes.

There is a definite link with advanced maternal age for reasons yet unknown.  However most babies with Down syndrome are born to women under the age of 35, as younger women have higher fertility rates.

What we do know is that no one is to blame.  Nothing done before or during pregnancy can cause Down syndrome.  It occurs in all races, social classes and in all countries throughout the world.  It can happen to anyone.

When was Down syndrome discovered?

It is believed that people with Down syndrome have always existed. However, it was not until 1866 that the English doctor, John Langdon Down published a description of the condition, which subsequently took his name.

In 1959 Professor Jérome Lejeune proved that Down syndrome is a chromosomal irregularity. Instead of 46 chromosomes usually present in each cell, Lejeune noted 47 in the cells of people with Down syndrome.  It was later determined that this additional chromosomal material results in the physical characteristics associated with the condition and the different course in development.