Down syndrome

What is Down syndrome?

Chromosomes contain all our genetic material. The correct number and arrangement of chromosomes are necessary for the development of a normal individual.

Down syndrome is an abnormality of the chromosomes in which the individual has 3 copies of chromosome 21, rather than the normal 2 copies. For this reason, the condition is also called Trisomy 21.

Children with Down syndrome are born with a characteristic physical appearance that includes slanted eyes, a flat nose on a round head, and abnormalities on the palms of the hands and the soles of the feet. They often have multiple birth defects and a degree of intellectual impairment.

Down syndrome occurs in fewer than one in 870 births. Older women have a higher risk of having a baby with Down syndrome, but women of any age can give birth to a baby with Down syndrome.

Screening tests for Down syndrome

Pregnant women can choose to have antenatal screening tests that identify those at higher risk of having a baby with Down syndrome. These include blood tests and a specialised ultrasound, known as a nuchal translucency scan.

The nuchal translucency ultrasound is undertaken at 11 to 13 weeks of pregnancy, and measures the width of an area on the back of the neck of the fetus. A maternal blood test (usually performed between 9 and 12 weeks) measuring 2 hormones (free beta hCG and PAPP-A) improves the accuracy of screening in the first trimester. These results can be combined to identify women at high risk.

High-risk women are then offered a confirmatory test. This test is invasive and involves either chorionic villous sampling (CVS) in the first trimester, or an amniocentesis in the second trimester. CVS involves removing some cells from the placenta, whereas amniocentesis involves testing the fluid that surrounds the fetus in the womb. These invasive tests carry a small risk of miscarriage (usually less than one per cent).

Health issues associated with Down syndrome

Down syndrome is often associated with several medical problems such as congenital heart disease, hearing loss, hypothyroidism (an under-active thyroid gland), cataracts and obstructive sleep apnoea. In addition, most children with Down syndrome will function in the lower range of intellectual abilities.

It is important that children with Down syndrome and their families receive support from their doctor, as well as other health professionals such as physiotherapists and speech therapists.

Last Reviewed: 21 December 2012


1. Mayo Clinic. Down Syndrome. Updated 7 April 2011. (accessed Feb 2013).
2. Lab Tests Online. Down Syndrome. Last reviewed 27 Aug 2011. (accessed Feb 2013).
3. Royal Australian & New Zealand College of Obstetricians & Gynaecologists. College Statement C-Obs 4. Prenatal screening tests for trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome) and neural tube defects. Review March 2010. (accessed Feb 2013).


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