Amniocentesis is usually carried out during weeks 15 to 20 of the pregnancy.

The procedure can be performed earlier than 15 weeks, but this is avoided as it may increase the risk of a miscarriage (loss of the pregnancy) or club foot in the foetus.

See complications of amniocentesis for more information about the risks of amniocentesis.

Occasionally, amniocentesis is carried out later in the pregnancy to test for an infection. For example, in rare cases it may be used to test a pregnant woman who has been exposed to the parvovirus B19, which causes slapped cheek syndrome (a childhood infection characterised by a bright red rash that develops on the cheeks).

Amniocentesis may also be recommended when there's a high risk of the unborn baby having a serious inherited condition.

This could be because:

• you've had a previous pregnancy with a foetal problem – for example, a baby born with a chromosome abnormality 
• you have a family history of a condition – for example, muscular dystrophy (an inherited condition that causes increasing muscle weakness)
• you're over 35 years of age – the risk of your child developing Down's syndrome increases with age
• an earlier antenatal screening test has suggested there may be a problem – for example, a high level of alpha-fetoprotein (a protein found in the blood) may indicate neural tube defects (birth defects that affect the spine and nervous system), such as spina bifida 

Amniocentesis or CVS?

Another diagnostic test known as chorionic villus sampling (CVS) can be carried out slightly earlier than amniocentesis (from about week 10 of the pregnancy).

CVS tests a sample of cells taken from the placenta (the organ that links the mother's blood supply with her unborn baby's).

With CVS, the risk of miscarriage is about 1-2%, which is slightly higher than the risk of miscarriage for amniocentesis. However, the test can be carried out earlier, so you'll have more time to consider the results.

Genetic counselling

If there's a risk you could pass a genetic condition on to your child, your GP or midwife can discuss appropriate tests with you and explain why they might be necessary.

In some cases, you may be referred to a genetic counsellor. Genetic counsellors are healthcare professionals who help people understand and deal with genetic conditions.

They'll discuss the likelihood of you passing certain genetic conditions on to your children. They'll also be able to give you advice about what to do when you get the results.

Find out more about genetic testing and counselling, or read What is genetic counselling and do I need a genetic counsellor? for more information.