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Down Syndrome—Trisomy 21

Trisomy 21, also known as Down syndrome, is a chromosomal condition in which there are three copies of chromosome 21 in the cells of the body instead of two. Down syndrome is the most common chromosomal disorder, affecting one in 550 newborn babies, in the absence of prenatal screening.

Characteristics of Trisomy 21

The term syndrome is used to describe a collection of distinctive characteristics that are often seen together in an affected individual. Babies with Trisomy 21 have recognisable characteristics referred to as Down syndrome.

Physical appearance

Babies with Down syndrome are likely to have many of the following characteristics (but not necessarily all of them):

  • Specific facial characteristics e.g. upward slanting eyes, small folded ears, a small mouth giving the appearance of a large tongue, a small flattened nose and a short wide neck.
  • Poor muscle tone making them appear floppy, as babies.
  • A single crease that goes directly across the palm of the hand.

Other health issues

Babies with Down syndrome are likely to have many of the following characteristics (but not necessarily all of them):

  • Intellectual disability.
  • Heart defects affect almost half of all babies with Down syndrome.
  • Intestinal malformations requiring surgery are found in approximately 12 per cent of babies with Down syndrome.
  • Vision problems are found in 60 per cent of children with Down syndrome and may require glasses, surgery or other treatments.
  • Hearing loss affects up to 75 per cent of children. Screening throughout early childhood is important to help with early diagnosis and treatment.

Most of the health problems associated with Down syndrome cannot be identified before birth but can be treated and the life expectancy for affected people is approximately 60 years.

Cause of Down syndrome

Down syndrome is caused by an extra copy of chromosome number 21and the incidence is equally divided across male and female babies. The chance of having a baby with Down syndrome increases with maternal age. The chance of a mother having a baby with Down syndrome at the age of 25 is approximately one in 1383. This chance increases to one in 84 at the age of 40.


Normally egg and sperm cells have 23 chromosomes each which join to form a future baby with 46 chromosomes (23 plus 23) in each cell. During fertilisation each of the chromosomes from the egg and the sperm fuse and separate. If this separation does not occur properly, a cell with three copies of the chromosome is the result. This cell starts rapidly copying itself to eventually form a baby with each of its cells having the extra copy of the chromosome 21. It is this extra chromosome that causes the intellectual and physical characteristics of Down syndrome.

Mosaicism and translocation

In one per cent of Down syndrome cases the extra copy of the chromosome 21 is not in every cell. This situation is called Mosaicism (mixture) and is likely to result in less severe characteristics of Down syndrome.

Very rarely, either of the parents are carriers of a copy of the chromosome attached to a different chromosome. This is called a translocation (rearrangement) and may not affect the parents. However future pregnancies are more likely to be affected when further rearrangements of the chromosomes may occur. In these instances genetic counselling is recommended.


Antenatal screening tests for Trisomy 21 have been developed and are available to women of any age. Screening tests include the first trimester combined screening test (involving the nuchal translucency ultrasound measurement and blood test between 11+0 and 13+6 weeks) and the second trimester triple test (blood test). The obstetric ultrasound between 18 and 20 weeks is not considered a screening test for Down syndrome as at leat half of the affected babies with Trisomy 21 will not be identified at this ultrasound assessment.


Trisomy 21 can be suspected prenatally (before birth) or after birth; however the diagnosis can only be made after chromosomal analysis. Diagnosis can be confirmed prenatally by chromosomal analysis on samples obtained following ultrasound-guided procedures such as chorionic villus sampling (CVS) or amniocentesis. For further information on prenatal testing please refer to Mater Mothers' Hospital's brochure: Prenatal diagnostic procedures.


Babies with Down syndrome have a variable prognosis which is determined by the health issues of the affected individual.

All families with an affected baby will be offered counselling by a neonatologist or baby doctor regarding the management of their baby.

Future pregnancies

There is approximately a one per cent chance of conceiving a baby with Trisomy 21 in a future pregnancy. This percentage is further increased if the mother is over the age of 40 or if the Down syndrome is due to a translocation.

Further information

Genetic Health Queensland (GHQ) provides a genetic counselling service three days a week at Mater's Centre for Maternal Fetal Medicine. A genetic counsellor is available to discuss all aspects of screening and diagnostic testing for Trisomy 13. To speak to a genetic counsellor please telephone 07 3163 1593.

For further information, we recommend you speak to your doctor. Alternatively, you may find the following websites useful:

For video resources on this topic in languages other than English, visit the Raising Children Network website.*

*Please note that Mater cannot guarantee the accuracy or appropriateness of information provided on third party websites.


Centre for Maternal Fetal Medicine

Level 7
Mater Mothers' Hospital
Raymond Terrace
South Brisbane Qld 4101
Phone: 07 3163 1896
Fax: 07 3163 1890
Website: www.matermothers.org.au

Mater acknowledges consumer consultation in the development of this patient information.
Mater Doc Num: PI-CLN-430109
Last modified 09/8/2017.
Consumers were consulted in the development of this patient information.
Last consumer engagement date: 28/2/2014
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