Family doctor


Pregnancy And Birth

AMNIOCENTISIS - a patient's guide


Pregnant women over 35 should consider whether to have an amniocentisis to test for fetal abnormalities. This article details the procedure, the risks and benefits.

What is amniocentesis?

Amniocentesis is a procedure where a small quantity of the fluid surrounding the fetus in the uterus is withdrawn through a needle passed through the abdominal wall of the mother.

When is it performed?

It is usually performed around the 15th week of pregnancy. Sometimes the amount of fluid (liquor) present at this time is not enough, and the procedure may need to be deferred for a week or two.

What are the indications?

The two most common indications for the procedure are to detect the presence of chromosome abnormalities in the fetus, or to detect abnormalities in the formation of the brain or spinal cord:

1. Chromosome abnormalities: every cell in the body contains genetic information (chromosomes). Cells shed from the skin of the foetus are found in the liquor surrounding it. By culturing these cells, the arrangement of the chromosomes making up the genetic information inside the cell can be identified. The most common chromosome abnormality is Down's Syndrome (mongolism), but there are other abnormalities.

2. Neural Tube Defects (abnormalities in the formation of the brain and spinal cord): certain substances in the liquor (alpha feto protein and acetylcholine sterase) are usually present in increased concentrations when the baby has a neural tube defect. The most common of these abnormalities is Spina Bifida. These protein concentrations are measured in the liquor. High quality ultrasound examination may be sufficient to exclude Neural Tube Defects. Amniocentesis for this indication is becoming less common.

Who should have the test?

Women who have previously had an infant with a neural tube defect have a chance of approximately 1 in 20 - 1 in 40 of having another affected infant. If you have had a baby with Down's Syndrome, the risk of having another affected infant depends on the chromosome pattern in the previously affected infant. If this was the most common form of Down's Syndrome (standard trisomy 21) then the risk is 1% plus your age related risk (see below).

If Down's Syndrome has occurred in other family members, the risks of recurrence may be very low and a blood test should be performed on the affected relative (this will often have been done already) before amniocentesis. If the affected family member had standard trisomy 21, the risk to your child is low and amniocentesis may not be indicated.

For a healthy couple, with no previous history of this abnormality, the risk of giving birth to an infant with a chromosome abnormality is related to the age of the mother. The father's age has only a very small effect. The risks are approximately as follows:

Woman's age







Risk - 1 in:







There are other indications for amniocentesis, but these usually involve rare problems.

The test will not diagnose many common abnormalities. It is well to remember that approximately 1 in 50 births in women of any age is associated with a congenital abnormality of some sort.

The figures above should be viewed in relation to the overall risk. At Fertility Associates we believe that the age at which women should be offered amniocentesis solely on the grounds of maternal age should be 35; however we are happy to discuss the situation with women of younger ages.

Is any preparation required?

No special preparation is needed. Sometimes some discomfort is apparent after the procedure, and your husband or a companion might be better to drive you home.

Is my partner welcome?

Partners and support persons are very welcome. It is very useful for them to be present at the discussion prior to the procedure. They will also have an opportunity to see the ultrasound scan performed prior to the procedure.

How is it done?

An ultrasound scan is performed to check on the baby's age and to choose a site for amniocentesis. Sometimes, if the pregnancy is not as far advanced as expected (and this bears no significance as to whether the fetus is normal or not), you will be asked to return at another time.

Local anaesthetic is placed in or on the skin and the amniocentesis needle is inserted through the skin into the pregnancy sac. A small amount of liquor is removed. This is not painful, but some discomfort following the procedure is not unusual.

What are the risks?

Current knowledge is that ultrasound scanning is harmless to both mother and babe. Approximately 1 in 200 of women who have an amniocentesis will miscarry because of it, but because spontaneous miscarriage is not uncommon at this stage of pregnancy, an absolute figure is difficult to assess. Damage to the fetus from the needle is extremely rare. If miscarriage does occur, it normally will happen within a week or two.

If your blood group is Rhesus negative, you will be given an injection to stop you forming antibodies against your baby's blood cells. This could only happen if your baby if Rhesus positive and some fetal cells are spilt into your circulation. This is very unlikely, but it is better to be safe than sorry.

What should you do after the test?

A quiet day is all we suggest. If there is vaginal blood or fluid loss, you should report it to your referring doctor right away, and rest in bed until the problem settles. Fluid loss is not too unusual, and is not necessarily a sign of miscarriage. Some soreness around the needle site occurs occasionally.

When are the results known?

The neural tube defect tests take about 3 days to be completed.

Your doctor will be sent the results.

The chromosome studies take two to three weeks, or sometimes a little longer. Very occasionally the amniotic fluid cells fail to grow and a chromosome result is not possible. If this happens, we would usually notify you about two weeks after the initial test, when it becomes obvious that growth is not occurring and we would normally offer to repeat the test.

Cell culture failure has no relationship to chromosome abnormalities.

The result will include the baby's gender. If you do not want to know it, it is important to let your doctor know.

What tests will be done on my pregnancy?

If you have an indication for chromosome analysis we will normally do the neural tube defect tests as well. Because of the expense of the chromosome test, if you are having amniocentesis to exclude neural tube defect, we will not normally culture cells for chromosome evaluation unless a separate indication exists.

See also:

Did this article meet your requirements/expectations?