Amniocentesis – an invasive test

When are women offered the test and why?

The test can be performed as early as 12 weeks or as late as 20 weeks, but it has been shown to carry a higher risk of miscarriage if performed earlier than 14 weeks gestation.

• It is common practice to offer you the test if you are over 35 years old. This is because you are at a higher risk of having a baby with the chromosomal disorder responsible for causing Down's syndrome.
• If an ultrasound examination has detected an abnormality associated with a genetic disorder, or if you have a family history that indicates that there is a risk of you having a baby with such a disorder, you will be offered the test at about 14-16 weeks gestation.
• It may be that an AFP blood test has indicated a risk of Down's syndrome - again, the test will be offered at this time, as there is enough amniotic fluid surrounding your baby to make it easier for the doctor to take a sample without putting your baby at risk.
• If, for some reason, your baby needs to be delivered several weeks/months before your due date, it might be suggested that an amniocentesis is performed in order to check that your baby's lungs are properly developed.

How is the test performed?

Amniocentesis is usually performed as an outpatient procedure. You will be offered either a local anaesthetic - administered by injection in the skin, or a topical anaesthetic - which can be rubbed into or sprayed onto your skin.

It isn't always necessary to have a local anaesthetic for this procedure, but you will be given the choice. A long, thin, hollow needle is then inserted through your abdominal wall - under ultrasound guidance - into the sac of fluid that surrounds your baby.

The ultrasound enables the doctor, who is performing the test, to identify the safest entry point for the needle, so that your baby or the placenta does not suffer any damage when fluid is removed (it has to be done in a clear area of the amniotic sac). It is this part of the procedure which takes up most of the time (anything up to 20-25 minutes), with just a couple of minutes to withdraw the fluid.

Is the test painful?

Everyone experiences the procedure differently. Some women report that they feel it with one pregnancy and not with another. If an anaesthetic is used, you may only feel a sharp, stinging sensation for a few seconds, and a sharp pain as the needle enters the amniotic sac.

Some women just report a feeling of 'pressure' in the abdomen, others report that it was 'no more than having a blood sample taken'.

What risks and disadvantages are associated with the test?

Amniocentesis, while certainly being the safest of the 'invasive' tests, can still lead to a spontaneous miscarriage in 0.5-1 per cent (that's 1 in 200) of women.

About 1 in 200 women undergoing the test develop an infection or some other complication as a result - this too, can lead to a spontaneous miscarriage. It is possible to have leaking of amniotic fluid - or even a small amount of blood - for a few days after the test, though this stops after a period of bed rest.

Some women can develop a high temperature, abdominal tenderness and some uterine contractions, which can also mean that there is infection.