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What is Down syndrome?
Down syndrome, also known as Down's syndrome or trisomy 21, is a genetic condition. Normally our genetic make-up consists of 46 chromosomes, 23 from our mother and 23 from our father. In the case of Down syndrome, an extra chromosome 21 is included in every cell, resulting in a total of 47 chromosomes. This additional chromosome accounts for some different physical and developmental characteristics and intellectual impairment. Not all characteristics are present in all people with Down syndrome and the level of physical, developmental and intellectual impairment can vary greatly.
Features of a person with Down syndrome
Normally at birth, a doctor is able to identify a baby with Down syndrome by their facial appearance. The face usually has a flat, rounded and small profile, the outer corner of the eyes slope upwards, the ears are small and the tongue is large and can stick out. Some other features can include:
- Delay in development (speech and language);
- Intellectual disability;
- Smallness in stature;
- Poor muscle tone;
- A single crease across the palm of the hand, and;
- A kind and loving nature.
Down syndrome is a genetic condition and is caused by an error during cell division. During fertilisation, or while an embryo is developing, cells divide abnormally to create extra chromosome 21 material. This additional genetic material can cause three different types of Down syndrome:
This is the most common form (92-95%) of Down syndrome.  This occurs when three copies of chromosome 21 are included in every cell. This occurs at the fertilisation stage.
Translocation Down syndrome
This is a rare form of Down syndrome (3-4%).  This occurs when a part of chromosome 21 breaks off and becomes attached (translocated) to another chromosome. This normally occurs after fertilisation, but in about one third of the cases, a parent also carries the same translocation.
Mosaic Down syndrome
This is the rarest form of Down syndrome (1-2%).  This occurs when only some cells have an extra copy of chromosome 21. This is due to an error that occurs after the fertilisation stage.
Anyone can give birth to a child with Down syndrome. Some risk factors include:
Increased maternal age
The age at which a mother gives birth can have a dramatic effect on the chance of having a baby with Down syndrome. For example, at age 20 the chance is 1 in 1500, at age 30 the chance is 1 in 900 and age 40 the chance is 1 in 85.  This correlation is related to the age of a woman's egg that becomes fertilised - older eggs have an increased risk of dividing abnormally.
Previous Down syndrome birth
If you have previously given birth to a child with Down syndrome, there is an increased risk (about 1 in 100) that it may occur again. 
Carrying a translocation of chromosome 21
If either parent is carrying a translocation of chromosome 21, there is a risk that they will pass it on to their children.
Methods for diagnosis
Down syndrome is often recognised during pregnancy, through initial screening tests followed by diagnostic testing. However, not every pregnant woman chooses to undertake this testing. Down syndrome is normally identifiable at birth by physical characteristics. To confirm this, a diagnostic blood test also can be taken.
Types of treatment
There is no cure for Down syndrome, however, early developmental intervention is suggested to ensure a fulfilling life. As there is an increased risk of other medical conditions in people with Down syndrome, it is important to have regular tests to detect these conditions early.
People with Down syndrome have an increased risk of developing other medical conditions. These can include:
The level of medical, developmental and intellectual impairments in people with Down syndrome can vary significantly. These impairments can affect the general health, life quality and life span of an individual with the condition. It is important to remember that although Down syndrome can affect development, it does not need to deter the achievement of a person. The best way to raise a child with Down syndrome is in a family and this is a key factor in obtaining the best outcome for the person. Children with Down syndrome are able to attend all forms of pre-school, primary and secondary school alongside other children. There are also many support services to help the family and child.
There is no way to prevent Down syndrome. Where possible, avoiding late-age pregnancy can reduce the chance of having a child with Down syndrome.