Amniocentesis

Amniotic fluid contains cells from the baby's skin and other organs which can be used to diagnose his condition. Amniocentesis is the name given to the procedure that withdraws this fluid from the uterus.

Why amniocentesis is done?

You'll probably want to have amniocentesis if you are over the age of 37, as the risk of chromosomal abnormalities (such as Down's syndrome) increases with age (see Older mothers). It may also be suggested after serum screening (Bart's triple test), or if a nuchal scan shows a risk of Down's syndrome. Amniocentesis can also reveal other important information, which may be sometimes helpful in determining the care and progress of your pregnancy.

At one time amniocentesis was used to check for metabolic disorders, but most of these are now diagnosed by CVS. It was also used to check the bilirubin content of the fluid to help work out if a Rhesus-positive baby had fetal anaemia and needed a blood transfusion while still in the mother's uterus, but this is now done by a Doppler scan.

What amniocentesis can reveal?

Where there is cause for concern, this test may show the following:

The sex of the baby: cells sloughed off by the fetus accumulate in the amniotic fluid. Under the microscope, male cells can be distinguished from female cells and the baby's sex determined. In gender-linked genetic disorders such as haemophilia, a male child will have a 50 per cent chance of being affected.
The chemical composition of the amniotic fluid: this can reveal metabolic disorders caused by missing or defective enzymes.
The chromosome count. This is determined by examining discarded cells. Any deviation from the normal chromosomal structure usually means that the baby has a disability.

How is amniocentesis done?

Amniocentesis is usually carried out at 16-18 weeks. Guided by ultrasound, the doctor inserts a hollow needle into the amniotic sac through the abdominal wall. About 10ml 1/3 floz) of amniotic fluid is usually withdrawn. This is spun in a centrifuge to separate the cells shed by the baby from the rest of the liquid. The cells then have to be cultured and it takes about three weeks for the results to come through, which can be a very stressful period for couples. Many women talk about putting their pregnancies “on hold” during this time, until the results confirm that the baby is unaffected.

Amniocentesis is always undertaken with ultrasound monitoring to guide the needle into the amniotic sac, so that neither the placenta nor the fetus is harmed. The risk of the procedure inducing a miscarriage is small - about two in 100. There's also the possibility of a small risk (less than one per cent) of respiratory difficulties in babies after amniocentesis.

Dr Miriam Stoppard

Posted 16.11.2010

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See more in the dossier: Your antenatal care