Amniocentesis

This is a test which may be offered to a woman whose age, family or medical history or blood test results suggest that she may have a higher than normal risk of having a baby with a specific abnormality.

Who may be offered amniocentesis?

1. Women in whom an ultrasound scan or blood test has suggested that the fetus is at increased risk of having a chromosomal abnormality such as Down syndrome.
2. Women who have already had a child with a problem such as Down syndrome or spina bifida.
3. Occasionally amniocentesis is done for other reasons. These include parents who are known to be at risk of having a baby with one of a number of rare ‘metabolic’ disorders or blood disorders such as thalessemia.

Why are women over 35 years of age offered amniocentesis?

As the age of a mother increases, the chance of her having a baby with a chromosome abnormality also increases. The commonest chromosome abnormality is Down Syndrome. Between 35 and 40 years of age, the chance of having a baby with a chromosomal abnormality is approximately 1 in 150. Beyond the age of 40 years this chance increases even further.

How and when is the test done?

While an amniocentesis can be performed at any stage in pregnancy, it is usually done at around 16 weeks.

Amniocentesis

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 withdraws a small amount of amniotic fluid (about 20 cc) from around the developing baby.

1. 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 known less common chromosome problems. Also, by looking at the chromosomes, the sex of the baby can be determined.

Why is an ultrasound examination needed before the amniocentesis?

The ultrasound scan will check that the pregnancy and placenta appear normal. It will also confirm the age of the pregnancy. At 16 weeks it is often possible to perform a good examination of the physical development of the fetus and to exclude many of the more severe structural abnormalities that are sometimes seen.

By finding the position of the baby and placenta (afterbirth) the doctor can avoid them while doing the amniocentesis.

What happens on the day of the test?

It is not necessary for the mother’s bladder to be full. However, having a little urine in the bladder may make it easier to see the baby and the lower part of the uterus.

After cleaning the skin, a needle is inserted into the womb, guided all the time by the ultrasound image. It then takes only about thirty seconds to draw up 20 cc of fluid needed into a syringe.

After the needle is removed, the baby is checked again.

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

After the procedure, she may return to her usual routine.

What complications can occur?

The risk of the test causing a miscarriage is low, not more than 1 in 200. It should also be noted that even if an amniocentesis is not performed there is still a risk ofup to 1% of a spontaneous miscarriage.

In about 0.5% of patients, there is some leakage of the amniotic fluid through the vagina in the 24-48 hours after the procedure. In almost all cases this stops on its own and does not cause any problems to the baby or to the pregnancy.

The test, like any other, may fail to give a result, the most common reason being insufficient material for the laboratory to examine. This is very uncommon.

How long will the results take?

The chromosome test for Down syndrome takes around two weeks because of the requirement to grow the cells.