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An amniocentesis is a procedure used to sample the fluid (amniotic fluid) that surrounds the baby in the womb (uterus). This fluid contains cells shed by the developing baby. After a small sample of fluid is collected, the baby's cells can be removed from the fluid and grown in the laboratory for testing 

What happens during the amniocentesis?

An ultrasound scan is done before, and during the test. The doctor cleans the woman's abdomen with antiseptic fluid and puts a very thing need into the uterus through the abdomen. The ultrasound is used to check the position of the needle, the position of the baby and determine if there is a suitable amount of amniotic fluid. 

A small sample of the amniotic fluid (about 15mls) is taken up through the needle. The woman may feel some slight discomfort during the test. The amniotic fluid contains cells shed by the devloping baby. These cells are allowed to grow and multiply in a laboratory until they are mature enough to test. Occasionally, the doctor is unable to obtain enough amniotic fluid on the first attempt. If this happens, another sample may need to be taken at this time. The results are usally available in about two or three weeks. 

When will the amniocentesis be done?

The test is usally performed between the 15th and 16th week of pregnancy. At this time, there is enough amniotic fluid to sample.

Why should parents' consider having an amniocentesis in pregnancy? 

You may consider having an amniocentesis in the pregnancy for one of the following reasons, if the woman: 

  • Is in her mid 30s or older since she has increased risk of having a baby with a chromosome problem based on her age alone. This applies to any pregnancy, even if it is the woman's first.
  • Has had a screening test such as a first trimester test that indicates that the baby has an 'increased risk' of having chormosomal problems. 
  • has had a previous child/pregnancy with a chromosomal problem.
  • Is at risk for having a baby with a genetic condition because either she and/or her partner may pass on particular faulty gene(s) to the baby. Not all genetic condtions that could affect the baby, however, may be diagnosed during pregnancy.
  • The woman's doctor or genetic counsellor will discuss with the parents the range of genetic condtions that can be identified using amniocentesis and what teach of these might mean for the parents and the baby. 

It is important to remember that a normal test result does not exclude every possible problem with the baby. Not all genetic conditions can be deteced by this test.

How reliable is an amniocentesis?

Chromosome studies on amniotic fluid following amniocentesis are very accruate. Very ocassionally, more than one amniocentesis is neccessary to obtain a diagnosis. This happens when the doctor is unable to obtain enough fluid on the first attempt, or when the laboratory is unable to make an analysis of the fluid. 

Does the amniocentesis identify all genetic conditions?

No. Although it is a diagnostic test, the amniocentesis has some testing limitations. Most major chromosomal problems can be identifed. Not every subtle chromosome abnormality, however, will be picked up. Genetic testing for specific genetic conditions can also be done in some cases where this is indicated, although, not all genetic conditions can be detected using prenatal testing. 

Your doctor or genetic counsellor will discuss the limitations of the amniocentesis test with you prior to the test being done 

Does the amniocentesis harm the mother or the baby?

There is a slight risk of miscarriage associated with having the amniocentesis test. This risk is in addition to the 'background risk' of miscarriage. There is a risk of miscarriage in every pregnancy due to natural causes. This risk is less at this stage of pregnancy than earlier on. It is important to ediscuss the background of risk of miscarriage with your doctor. This risk of miscarriage due to amniocentesis is less than 1% (occuring in less than 1 in 100 pregnancies tested with amniocentesis). The specific risk figure depends on the experience of the doctor doing the test and the difficulty he/she has in getting the sample. It is therefore important than an amniocentesis is only carried out by a doctor experienced in this technique. It is not uncommon to have cramping on the day of the test. A small number of women may experience bleeding and some leaking of amniotic fluid. The woman's doctor should be contacted if this occurs. 

What happens if the result of the amniocentesis shows the baby has a problem?

Most often, the baby will not have the particular genetic condition for which it was tested. In those few cases where the test does show that the baby has a problem, the parents will be given as much information as possible to enable them to make a decision about where to continue the pregnancy. If parents decide to terminate the pregnancy, details of the procedure required (including need for general anaesthetic and time spent in hospital) will be discuessed. The method of termination will vary depending on how advanced the pregnancy is and the policy of the particular hospital or clinic. All aspects should be fully discussed with your doctor or genetic counsellor. Parents will be offered support and information before, during and after making decisions of this kind. Genetic counselling is recommended. 

What other tests are available later in pregnancy?

Even if the amniocentesis shows that the baby does not have certain problems, a detailed ultrasound is usally oferred between 18 and 20 weeks of pregnancy to check the baby's physical development. 

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