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What is it?

Down syndrome is the most common form of congenital intellectual disability. It is caused by an extra copy or piece of chromosome 21 in all or most of the person’s cells. Down syndrome is also known as astrisomy 21 because ther are three copies of chromosome 21 rather than the usual two. The characteristic features of Down syndrome are due to an error in cell division that occurs before, or shortly after, conception. This error affects the physical and mental development of a 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 to form a complete set of 46 when an egg is fertilised (half from each parent). In most cases of Down syndrome, random chance leads to an extra copy of chromosome 21 appearing in either the egg or sperm. This extra copy becomes part of the fertilised egg and is replicated in the embryo’s cells. This form of trisomy 21, accounts for about 95% of cases.

The error may also occur after conception, in the developing embryo. During development, some cells may replicate 47 chromosomes, while others have 46. This  is called mosaic trisomy 21.

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

Individuals with additional chromosome 21 genetic material, regardless of the cause, will develop some of the features of Down syndrome. The most common features associated with Down syndrome include: low muscle tone ( babies appear "floppy"), flat facial features, with a small nose, almond-shaped eyes, small skin folds on the inner corner of the eyes, small and abnormally shaped ears, single deep crease across the centre of the palm. About half of the babies with Down syndrome have heart defects and approximately 1 in 10 will have gastrointestinal problems.
Approximately 1 in 1,100 babies in Australia are born with Down syndrome. This is less than the worldwide birth rate of around 1 in 700 due to the high termination rates in Australia. The chance of having a baby with Down syndrome increases significantly with a woman's age. Among pregnant women aged 30 years, 1 in 890 will have a fetus with Down syndrome. By 35 years of age, the chance of having a fetus with Down syndrome is 1 in 355 and by 44 years of age, the chance increases to 1 in 35 pregnancies.

Most (66%) fetuses with Down syndrome occur in women under 35 years of age.

Last Review Date: Decemeber 30, 2018

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