Trisomy is the term given to describe an abnormal number of chromosomes within the cells that form an individual.
The human body normally contains 46 chromosomes per cell, which group into 23 pairs. When trisomy occurs, the individual carries an extra chromosome in one of these pairs, making a total of 47 chromosomes.
Because the extra chromosome can join any of the pairs, there are different types of trisomy, each of which will generate a defect within the individual. The severity of the defect will depend on the type of trisomy.
For example, in trisomy 21 (known as Down's syndrome), the extra chromosome is present in the 21st pair, meaning that there are three 21st chromosomes, as opposed to the usual two. Down's syndrome causes behavioural and learning difficulties, although the baby is normally capable of survival and, in most cases, will have a quality of life.
However, trisomy 18 (where the extra chromosome has joined the 18th pair) causes such severe mental and physical problems that most babies are unable to survive beyond one year of life. This is known as Edward's syndrome.
It is possible to undertake tests on the foetus to determine the presence of a trisomy, and is normally done through amniocentesis or chorionic villus testing.
While there is a small risk to the unborn baby, both tests provide incredibly accurate results, allowing both the woman and her doctor to decide on a necessary course of action if a trisomy is detected.