Amniocentesis
Amniocentesis is a special test that is used increasingly in pregnancy to detect certain abnormalities that may be present in the developing baby.
It is usually performed in the first half of pregnancy around the 15- to 16-week stage. The test involves the removal of a sample of amniotic fluid (the ‘waters’ around the baby) through a fine needle. This is done under the guidance of ultrasound, which allows the doctor to see the position of the baby and its placenta, and therefore avoid damaging them with the needle.
Several tests can be done on the amniotic fluid. The amount of a special protein is measured. A raised level can be a warning that the baby's spine has not developed normally.
Cells from the baby are present in the fluid and they will be examined to make sure the chromosomes are normal. Chromosomes carry the genes that determine everything about a person's physical characteristics. Certain abnormalities occur when the number of chromosomes is abnormal. The most common of these is known as Down syndrome.
It is also possible to determine the sex of the baby from its chromosomes. Some parents are very interested to know this in advance, while others prefer the excitement of finding out at birth.
Amniocentesis is not usually painful. There may be slight discomfort but rarely any more than that. Like all things, it does carry a small risk. An estimated one-in-200 risk of miscarriage is the main concern.
Not all pregnant women need amniocentesis. It is most often carried out on women over the age of 37, because the chances of a baby having abnormal chromosomes increases with its mother's age.
Women who have had a previous baby with problems, and those who have certain diseases in their families, may also be offered an amniocentesis.
Last Reviewed: 01 May 2002
