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Genetic screening involves testing people or groups of people for the presence of a particular allele or other genetic abnormality.

One type of genetic screening is amniocentesis.

A needle is inserted into the amniotic fluid that surrounds the foetus and fluid is withdrawn.

Foetal cells in the fluid are then examined for the presence of genetic abnormalities such as Down syndrome and cystic fibrosis.

As amniocentesis carries a 1% risk of miscarriage, it is usually only offered to pregnant women who are at a higher risk of having a child with a genetic abnormality.

Mothers with a greater risk include:

• Those who have previously carried a foetus with a genetic abnormality.
• Those who have a family history of a genetic condition.
• Older mothers.
• Those where possible problems have been identified in an earlier medical examination, for example a blood test.

When screening for Down syndrome, pregnant mothers are offered a blood test between 10–14 weeks of pregnancy.

The blood test determines if the possibility of having a Down syndrome child is raised.

Amniocentesis is normally then offered to those mothers who appear to have a higher risk.

While not as accurate as amniocentesis, the blood test carries no risk to the foetus or the mother; it simply identifies those women who may wish to take the riskier amniocentesis procedure.