Down syndrome (DS) is a congenital condition caused by an extra copy or piece of chromosome 21 in all or most of the affected person’s cells. It is a group of signs, symptoms, birth defects, and complications that arise from an error in cell division that occurs before, or shortly after, conception. This error has a widespread effect on the physical and mental development of the affected person.

Chromosomes hold the body’s genetic blueprint. Most cells in the body contain 22 pairs of chromosomes and a 23rd set of either XX (in females) or XY (in males) for a total of 46 chromosomes. Reproductive cells, eggs and sperm, contain a single set of 23 chromosomes that combine when an egg is fertilised to form a new set of 46 in the fetus (half from each parent). In most cases of Down syndrome, random chance leads to the insertion of an extra copy of chromosome 21 in either the egg or sperm. This extra copy becomes part of the fertilised egg and is replicated in all of the embryo’s cells. This form of Down syndrome is called trisomy 21, and it accounts for about 95% of DS cases.

The error may also occur after conception, in the developing embryo. As the fetus grows, some cells may have 47 chromosomes, while others have 46. This form of Down syndrome is called mosaic trisomy 21.

In another rare form of Down syndrome called translocation trisomy 21, a piece of chromosome 21 becomes joined to another chromosome before or at conception. Even though the fetus has 46 chromosomes, it still has an extra portion of chromosome 21 in its cells.

All individuals with additional chromosome 21 genetic material, regardless of the cause, will develop some of the features of Down syndrome.

About 1 in 700 babies in Australia are born with Down syndrome. The risk of having an affected baby increases significantly as a woman ages. According to the US National Institute of Child Health & Human Development, the risk increases from less than 1 in 1,000 in women under 30 to 1 in 400 by age 35 and to 1 in 12 by the time a woman is 49 years old. However, since younger women have the greatest number of babies, the majority of those with Down syndrome, about 75%, will be born to women under 35.