Amniocentesis

This is a test which may be offered to a woman who has a higher than normal risk of having a baby with a specific abnormality. You and your doctor will decide whether it is appropriate for you to have this test.

Who may be offered amniocentesis?

1. Women with an abnormal nuchal translucency test or maternal blood triple test.
2. Women who have an abnormality on the mid pregnancy ultrasound at 18 – 20 weeks.
3. Women aged 35 and above.
4. Parents who are known to have a chromosomal abnormality themselves or who have a family history of a genetic disorder.

An amniocentesis can be performed at any stage in pregnancy after 15 weeks.

How is the test performed?

The mother will have an ultrasound examination, which will confirm the dates of her pregnancy and show the position of the baby and placenta.

Using a slender needle, which is guided by ultrasound, the doctor can locate a pocket of amniotic fluid separate from both the baby and placenta and withdraw a small amount of fluid (approximately 20-30mls). It only takes about thirty seconds to draw up the small quantity of amniotic fluid.

Normally the fluid is used to carry out two tests.

1. There is a quick result called a fast FISH (Flourescent In-Situ Hybridisation), which is a preliminary result only. If this is abnormal the final result is required to confirm this.

2. A test is carried out on the cells which are present in the fluid. Using special procedures in the laboratory, the chromosomes in these cells can be examined and in this way it is possible to find out if the baby has Down syndrome, or any of the other less common chromosome problems.

After the needle is removed and the baby is checked again, the mother may leave the room immediately to empty her bladder and dress.

Any discomfort felt is usually minor. Almost without exception, women experience far less discomfort from the test than they had expected.

The doctor who is looking after you and your pregnancy will tell you the results of the amniocentesis. In most cases, he/she will also be able to tell you the sex of the baby if you want to know.

There are a number of other tests which can be carried out on the fluid but these are only performed when the parents are known to be at risk of having a baby with a certain genetic condition.

A comprehensive assessment of the baby’s structures cannot be performed at 15 weeks. Your doctor may therefore arrange for another scan at around 18-20 weeks when details of the fetal development are more easily seen.