What is Down Syndrome?

Down Syndrome is the commonest cause of developmental disability.

Babies with Down’s are born at the same rate into families from all backgrounds, and commonly where there is no risk factor. However, it has long been known that the chance of giving birth to a baby with Down syndrome increases as the mother gets older. Eighty to 100 Down’s affected pregnancies occur each year in WA, including live born, stillborn and interrupted pregnancies (not including pregnancies which miscarry).

An individual with Down Syndrome can have a range of developmental difficulties. This includes delay in motor skills (such as learning to walk) and in cognitive skills (such as speech and short-term memory). People with Down Syndrome have a higher rate than the general population of certain physical problems. This includes poor muscle tone, hearing and vision defects, heart abnormalities, infections, gastrointestinal blockages, leukaemia, epilepsy and thyroid disorders. They have a characteristic facial appearance which is generally recognisable across racial groups.

The range of abilities and disabilities, characteristics, interests and achievements among people with Down Syndrome vary widely, as with the rest of the population. Typically, mental retardation is in the mild (IQ 50–70) to moderate (IQ 35–50) range. People with Down’s are often regarded as having social ability above their IQ. With good health care, people with Down Syndrome can now expect to live to 50-60 years of age.

Practical measures can be offered, such as surgery for specific defects. Research is underway to help find treatments for difficulties common in Down Syndrome people, such as gene defects of immunity, but is so preliminary that parents should not have expectations in the near future.

What causes Down syndrome?

The most common cause (in about 95%) of Down Syndrome is Trisomy 21, when an infant is born with an extra copy of the very small number 21 chromosome. Chromosomes are the structures that contain the genetic information that we need to grow and develop. Each cell of our body normally contains 23 pairs of chromosomes, so 46 chromosomes total, with one of each pair inherited from each parent.

The extra chromosome 21 is the mechanism where the chance of a Down’s baby increases with the mother’s age. It is this extra genetic material in infants with Down Syndrome that impairs physical and cognitive development.

Related Information

In a small number of cases, there are other mechanisms for Down’s, such as translocation or duplication of the chromosome 21. These are identified by the chromosome tests, but the effect of the extra chromosome material is essentially the same as Trisomy 21.

Usually, an individual either has Down Syndrome or doesn’t, and the extra chromosome material is present in most of the cells of the body. However, in 1 to 2%, there is a mosaic form for Down’s, where the condition is only present in some cells. The condition is then likely to be milder in the person, and also less likely to be picked up by screening tests. This includes the newer non-invasive prenatal diagnostic tests, NIPT.

what is down syndrome

Options for testing for Down syndrome

Available tests include screening tests (which are most appropriate for the majority of pregnant women who do wish to evaluate their risk), near-diagnostic tests (NIPT) and diagnostic tests (yes/no tests are especially suited to higher risk situations, meaning CVS and Amniocentesis tests). See the 12-week scan page, and NIPT, CVS and Amniocentesis for further information.

No one has to have screening. However, a range of other problems can also be found at the 12 week scan and at other stages. Therefore, it is reasonable to request the 12-week scan for general risk of abnormality, even for parents who choose not to have Down’s screening.

Families have to make their own decisions about care and what they are prepared for. The decisions can be very different if a problem is recognised during the earlier part of pregnancy, from the finding of Down Syndrome in a baby once it is born.

For instance, even parents who dearly love one child with Down Syndrome may not feel the family would cope as well with another. You may need advice from several sources, but you and your own family are ultimately the ones most involved and responsible. You live with the results of your decisions, even if other people around you may try to guide your decisions with their own value set.

You do not have to be sure of the choices you would make in order to undergo screening tests, but you should think about the sequence of testing. This is because you cannot pre-judge the result. Some parents use a different cut-off for the difference between high-risk and low-risk (commonly 1 in 300 is used), if they are more worried about risk of miscarriage, than risk of Trisomy 21. Other chromosome conditions may also be identified by screening tests, including ultrasounds or the diagnostic tests.

Try not to get bogged down with decisions about Down’s tests. The only reason your Doctor or carer advises you to consider tests in the first place, is to give you options. Many other and more severe medical conditions exist, some of which have no available prenatal tests, so no decisions are to be made, so paradoxically, there is less worry about them.

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