Down's Syndrome

Down's syndrome is due to the presence of an extra chromosome number 21, which means that instead of having 46 chromosomes there are 47. The incidence of Down's syndrome increases with increasing maternal age, and paternal age can also play a part. It is usually due to failure of the process called meiosis, where the paired chromosomes are separated, so that we receive one chromosome from each parent - the pair of chromosomes number 21 don't separate, so that child receives two from one parent and one from the other. In some cases it is due to a translocation of the chromosome on to another - usually chromosome number 14 - in which case there is an increased risk of recurrence in subsequent pregnancies.

While some people with Down's syndrome are of normal intelligence, there is a high incidence of learning difficulties. There are pre-natal tests available to detect the presence of Down's syndrome in a foetus, including amniocentesis (with drawing a sample of the fluid, called amniotic fluid from around the foetus and culturing cells so that the chromosomes can be examined), chorionic villus sampling (which involves taking a biopsy from the placental tissue, which is partly made up the foetal cells). There is a blood test which measures the levels of three chemicals (and therefore called the triple test) which can be performed at about 16 weeks of pregnancy. Comparing the levels of the chemicals with the reference range for mothers of the same age may give an indication as to whether or not there is a risk of Down's syndrome, and whether amniocentesis or chorionic villus sampling is necessary. In some cases ultrasound scanning of the foetus may indicate the presence of Down's syndrome.

Despite the risk of learning difficulties, many with Down's syndrome lead a normal life into late adulthood.
 

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