Amniocentesis is the process of taking a small amount of the amniotic fluid from around the developing fetus to allow it to be sent to the laboratory for testing. It is used to detect Down Syndrome or other chromosome abnormalities in the fetuses of couples who are at increased risk of having an affected child. These include couples in which the mother is aged 35 years or more, or have already had a baby with a chromosome problem, or who have an inherited chromosome problem in the family. There are other less common reasons for amniocentesis as well.

Amniocentesis is an out-patient procedure carried out between 15 and 18 weeks of pregnancy. Partners are welcome to attend. It takes about 30 minutes and no special preparation is necessary. Most of this time is spent being scanned and setting up equipment; it takes only 30 seconds to obtain the sample of amniotic fluid. You will need to know your blood group and if you are rhesus negative you will be given an injection after the procedure. The Consultant will explain why this is necessary.

The fluid is sent to CGS in London for testing. Chromosome tests and spina bifida tests are performed. Two chromosome tests are carried out, one on the DNA from the fetal cells, giving a provisional result in 72 hours the second on cultured cells, giving a final result in 10-14 days. The sex of the fetus can be revealed by the chromosome tests and parents can choose whether or not they wish to have this information.

The spina-bifida test measures the amount of a protein (AFP) in the amniotic fluid and it takes about a week for the results to be made available. If there is a known increased risk that the fetus might have a neural tube defect, a further test will look for the presence of another substance in the fluid, acetylcholinesterase (AChE). This is usually present when the fetus has a neural tube defect but is absent from normal amniotic fluids.

Amniocentesis is a safe procedure but there is a small risk of miscarriage. About one in one hundred women (1%) will miscarry her fetus as a result of amniocentesis. If the fetus is shown to have an abnormality, you will be told what the problem means and if treatment is available. You and your partner can then decide whether or not you wish to continue the pregnancy. Couples considering amniocentesis should think about whether or not they would wish to continue with the pregnancy if an abnormality were detected but do not have to make up their minds immediately.

Clinical procedure notes:

• Ultrasound will be used to find the position of the fetus, placenta and a suitable pool of amniotic fluid.

• An area of skin on the abdomen will be cleaned with antiseptic and local anaesthetic injected or sprayed on the skin.

• A fine needle will be guided through the mother’s abdomen into the amniotic fluid. Ultrasound is used to guide the needle. Most women say afterwards that the procedure is not painful and the thought of it was much worse than the actual test.

• The movement of the needle will be checked by ultrasound throughout the procedure and you may be able to watch what is happening on the video screen, if you wish to.

• Eighteen ml (3 teaspoonful) of amniotic fluid are removed and the needle withdrawn. You will be asked to rest at home for the rest of the day and not to carry out vigorous activity for a further two days.

• The whole procedure takes about 15 minutes.