Amniocentesis

Reviewed by Dr Philip Owen, consultant obstetrician and gynaecologist

What is amniocentesis?

Amniocentesis is one of several diagnostic tests that can be carried out during pregnancy. It is used to detect any possible chromosome abnormalities in the unborn child that may cause Down's syndrome or other congenital problems.

In amniocentesis, a sample of the amniotic fluid that surrounds the foetus is removed and analysed. This test is performed from week 15 of the pregnancy onwards.

When is amniocentesis offered?

Amniocentesis is offered during pregnancy when:

a woman is considered at high risk because of a serum screening test (blood test), which measures the levels of pregnancy hormones in her blood. By combining these results with the mother's age, it is possible to calculate the risk of the child having Down's syndrome.

a woman is considered at high risk because she has previously had a pregnancy affected by a chromosomal or genetic disorder.

an ultrasound examination has detected features or abnormalities indicating an increased risk of a chromosome abnormality.

a woman requests it because she is concerned that her baby may have a chromosome abnormality.

amniocentesis is occasionally required in the investigation and management of other conditions in pregnancy such as rhesus diseases. In these circumstances, the amniotic fluid is analysed for substances in the fluid itself and not specifically to look at the baby's chromosomes.

How is amniocentesis carried out?

Using ultrasound to guide the way, a fine needle is inserted through the mother's abdomen and into the fluid surrounding the foetus. About 20ml of the amniotic fluid is removed and sent to a laboratory for analysis. The procedure usually takes 5 to 10 minutes and the mother will feel only slight discomfort. The foetus will be unaware of the procedure.

After the procedure, the mother should rest for the day. She may feel slight cramping. Bleeding and leaking of fluid from the vagina is not normal and she should contact the hospital if this happens.

If her blood group is rhesus-negative (one in eight women have this blood group), she will be given an injection of human anti-D immunoglobulin into a muscle of her arm or leg. Anti-D immunoglobulin is a substance administered to rhesus-negative women when they have been potentially exposed to rhesus-positive blood cells, for example after giving birth, a threatened miscarriage, or amniocentesis. Its purpose is to prevent the mother developing antibodies against the foetal blood cells, which can cause severe jaundice or even foetal death. The use of Anti-D immunoglobulin has been one of the success stories of the last 20 years.

How long will it take to know the result?

A preliminary result may be available within 48 hours and the final result will be known in two to three weeks.

Is amniocentesis safe for the mother and baby?

Amniocentesis is not risk-free and carries a slight risk of causing a miscarriage. Between 1 in every 100 and 1 in every 200 amniocentesis procedures will cause a miscarriage, usually within three weeks of the procedure.

Before deciding upon screening and diagnostic tests the mother should discuss any concerns she may have with her obstetrician. Having information about the baby's chromosomes has to be balanced against the small but real risk of losing the pregnancy.

How reliable is the result from an amniocentesis?

Very few things in life are guaranteed 100 per cent reliable. Amniocentesis comes very close, with a certainty of 99.9 per cent in identifying chromosome abnormalities.

What happens if the test shows abnormalities?

There is no treatment for chromosome abnormalities. If a diagnosis is made before birth, and depending on the nature of the abnormality that has been detected, the woman will usually be offered a termination of pregnancy.

The decision whether or not to continue with the pregnancy always rests with the parents and some parents may decide to continue.

Before an amniocentesis test is carried out, it is therefore important for the expectant parents to consider what difference an abnormal result would make to their wish to continue with the pregnancy.

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