Amniocentesis is usually performed during weeks 15 to 20 of the pregnancy. The procedure can be performed earlier than 15 weeks but, if possible, this is avoided because it may increase the risk of causing a miscarriage or club foot in the foetus (see Amniocentesis - complications).
Occasionally, amniocentesis is performed 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 (this causes slapped cheek syndrome – a childhood infection that causes a bright red rash on the cheeks).
Amniocentesis may be recommended when there is a high risk of the unborn baby having a serious inherited condition. This could be because:
- you have had a previous pregnancy with foetal problems, such as a baby born with a chromosome abnormality
- you have a family history of a condition, such as muscular dystrophy (an inherited condition that causes increasing muscle weakness)
- you are over 35 years of age, which means that you have an increased risk of your child having Down's syndrome
- an earlier antenatal screening test has suggested that there may be a problem – for example, a high level of alpha-fetoprotein (AFP – a protein found in the blood) may indicate neural tube defects, such as spina bifida (birth defects that affect the spine and nervous system)
Amniocentesis or CVS?
Another diagnostic test, called 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 that are 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 around 1-2%, which is slightly higher than the risk of miscarriage for amniocentesis. However, because the test can be carried out earlier, you will have more time to consider the results.
See the Health A-Z topic about CVS for more information about this procedure.
Genetic counselling
If you are at risk of passing a genetic condition onto your child, your GP or midwife can discuss any appropriate tests with you, and explain why they might be necessary.
In some cases, you may be referred to a genetic counsellor (a healthcare professional who helps people understand and deal with genetic conditions). They will discuss the likelihood of you passing on certain genetic conditions to your children. They will be able to advise you on what to do when you get the results.
For more information, see the Common health question called why do I need a genetic consultation?
