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  Logo Ultrasound
Chorionic Villus Sampling
 
What is amniocentesis?

An amniocentesis involves taking a small sample of amniotic fluid (water) from around the developing fetus. A needle is first passed through the pregnant woman's skin, then through the wall of the uterus (womb), and on the amniotic fluid. Approximately 15 mls (3 teaspoonsful) of fluid is taken, this amount being small compared with the total amount of fluid present at the time of the test at 15 weeks.

Genetic Counselling

Genetic Counselling maybe offered by your own doctor or alternatively is available at the practice prior to C.V.S. and other procedures. This is to help you understand both the test itself, plus birth defects and inherited conditions in general; it also allows relevant family medical and pregnancy information to be checked to assess your specific risks. You will then be informed of the risks, benefits and limitations of testing to you. You can choose the course of action that is appropriate to you in view of your own specific family needs and goals.

We recommend counselling on a day prior to the procedure to enable you to better consider your options, but it is available also at the practice on the day of the procedure.

Who is offered amniocentesis?

Women of 37 years and over at the estimated time of delivery are routinely offered testing in Victoria. Some younger women may also be offered it (in many other states of Australia and other countries it is offered at 35 years and sometimes even younger).

Other reasons for offering an amniocentesis include:

  • Women who already have a child with chromosomal problem such as Down Syndrome,
  • Women who know that an inherited disorder runs in their family or in the family of their partner.
  • Women who have an abnormal 'serum screen' test - a blood test for determining who is at increased risk of a fetus with Down Syndrome or spina bifida,
  • Women who have a child with a neural tube defect such as a spina bifida
  • Occasionally amniocentesis is performed for other reasons. These include couples who are known to have a chromosome abnormality themselves, plus those known to be at risk of having a baby with one of a number of other rare disorders.

Why are older women offered amniocentesis?

As the woman's age increases her chance of having a baby with a chromosome abnormality, such as Down Syndrome, also increases. The chance of Down Syndrome is the same whether or not this the first child.

In each human cell there are 46 chromosomes, each of which is paired giving 23 different chromosome pairs. The only exceptions are the egg and sperm which each have 23 chromosomes. Each chromosome contains thousands of genes which control an individual's growth and development. Birth defects may result if there are too many or too few chromosomes. The best known chromosome abnormality in newborn babies is an extra chromosome 21 so that the baby has 47 chromosomes in the cells instead of the normal 46. This causes a condition known as Down Syndrome (previously known as mongolism). A chromosome abnormality cannot be cured or corrected.

The chance of having a live born baby with Down Syndrome when you are aged 37 years is approximately 1 in 220. The chance at 40 years is approximately 1 in 90. The chance of having a live born baby with any chromosome abnormality is about double these figures. With increasing age there is no increase in the chance of having a baby with spina bifida. Most other abnormalities also do not depend on the age of the pregnant woman so that if the chromosomes are normal then most of the increased risk in 'older' pregnant woman has of delivering a baby with an abnormality is removed.

It is important when looking at the chance of a baby having a chromosome abnormality to keep the figures in perspective:

  • approximately 4% of babies have some abnormality at birth and most of these have normal chromosomes.
  • chromosome abnormalities are rare compared to many of the other risks of pregnancy, for example 5% of babies are born early.
Amniocentesis
Amniocentesis: ultrasound is used to guide the needle into a pocket of fluid

What type of disorders can be detected by amniocentesis?

The abnormalities looked for include:

  • Chromosome abnormalities such as Down Syndrome
  • Neural tube defects (ie spina bifida or anencephaly),
  • Some inherited disorders in those couples who have been shown to be at high risk, eg cystic fibrosis.

Amniocentesis is not a general test for the fetus - it tests for only very particular problems, usually only chromosomal abnormalities and spina bifida.

When is amniocentesis carried out?

We carry out amniocentesis at approximately 15 weeks. It is probably safer to wait until this time although 'early amniocentesis' is offered at some centres prior to 14 weeks.

Why is an ultrasound examination needed before the amniocentesis?

An ultrasound examination will first be carried out to confirm the age of the pregnancy, and to make sure there are not twins, and to check for physical abnormalities of the fetus.

By locating the position of the fetus and placenta, steps can be taken to avoid them during the amniocentesis. Ultrasound is used throughout the amniocentesis procedure to first guide the needle to the safest position, and then to keep the needle away from the fetus.

Your doctor may arrange for another scan at around 18 to 20 weeks when a more detailed check is possible.

What should I do before the Test?

It is not necessary to have a full bladder, however having some urine in the bladder may make it easier to examine the fetus and lower part of the uterus.

You should expect to be at our practice for approximately 1.5 hours.

What happens on the day of the test?

When the scan has been completed the amniocentesis is performed without you having to move from the examination couch.

After cleaning the skin, a very fine needle is inserted into the uterus watching the needle all the time on the ultrasound screen. It then only takes about 30 seconds to draw the required 15 mls (3 teaspoons) of fluid into the syringe. The needle is removed and the fetus is again checked. After waiting for about 20 minutes you are then able to return home.

Patients usually feel far less discomfort from the test than they expected - many indicate that it is not even as bad as a simple blood test. If you are worried about the test nitrous oxide, the gas used in labour, is available, although most patients find they do not need it.

What is done with the cells?

Amniotic fluid contains cells which have been shed by the fetus. They are allowed to grow in the laboratory until there are enough to test the chromosomes. Each of the chromosomes from a number of cells is carefully examined to check that the correct number of cells is carefully examined to check that the correct number are present ad that the appearance and length of each chromosome is normal. If the fetus has Down Syndrome (mongolism) then there is an additional chromosome 21 present. The sex can also be determined by looking at the chromosomes - you can find this out if you ask your doctor.

The laboratory will usually have the chromosome result available in 2 weeks - your doctor will be informed and he or she will pass it onto you. The results of the test for spina bifida will be available at the same time.

How will I feel afterwards?

While you will be quite capable of driving yourself home after the test, most people appreciate having a companion to drive home. It is recommended that you rest for the remainder of the day although there is no reason for you to go to bed. There need be no further limitation on your normal routine after this time.

If you have to travel more than one to two hours to get home, it is preferred that you stay in Melbourne overnight afterwards.

Occasionally after the test patients may have either mild discomfort or some bruising under the skin but neither causes any problem to the pregnancy.

What complications can occur?

In a study Lachlan de Crespigny carried out, including his one other doctor's results, the miscarriage result was not demonstrably higher than those who did not have the test performed. The risk of the test causing miscarriage therefore is very low, a best estimate is 1 in 200 or less. There is also about a 1 in 100 chance of having some leakage of amniotic fluid through the vagina for one to two days after the procedure. This usually stops if you rest and it can be expected to cause no problem to the fetus or the pregnancy.

If your blood group is Rh negative, anti-D is usually given only if the needle passed through the placenta. Direct injury to the fetus by the needle has not been reported after any of our patients. Rarely have any complications to the pregnant woman or other complications to the fetus been reported.

Couples often ask when a miscarriage would occur. This is a difficult question because at least two out of three miscarriages after amniocentesis are totally unrelated to the test itself and would have occurred anyway. However, if you have had no problem by three weeks then any complications is very much related to the experience and expertise of the operator.

The test, like any other, may fail because no specimen is obtained or because the laboratory are unable to grow cells. Both of these are very uncommon.

Incorrect results are extremely rare. Occasionally uncertain results may occur - these may need a further test such as a sample of fetal blood to sort them out.

What should I look for after the test?

If you lose blood or clear fluid from the vagina after the test then you should contact your own doctor.

What if the amniocentesis shows that an abnormality is present?

Those few patients in whom the tests show that the fetus has an abnormality will be given information so that they can make a choice about whether to continue the pregnancy. It is still possible to perform a termination of the pregnancy at this stage. Counselling is available to help couples make their decision.

Should I have an ultrasound examination later in the pregnancy?

This depends on how well the fetus can be seen at the time of the amniocentesis. Usually details of the structure are difficult to see at this early stage, so your doctor may recommend a repeat examination later.

What does the test cost?

Details of charges are available at our practice. Please note that the laboratory will send a separate account for the analysis of the specimen plus any necessary blood test. Medicare will cover part of the cost of these accounts.

Summary

In experienced hands, amniocentesis is a straight forward procedure which is well tolerated by patients with special indications, the commonest being the age of the pregnant woman. The test checks for Down Syndrome and other chromosome abnormalities, and also for spina bifida. It is important to realise that while the combination of the ultrasound scan and amniocentesis goes a long way to ensuring that the fetus is normal, it does not guarantee that every possible abnormality has been excluded.

If you would like more information, please ask. If you would like more written information we have pamphlets available on ultrasound and other procedures. For more detailed information, you may purchase a book Lachlan has written, "Which Test For My Unborn Baby? A Guide to Prenatal Diagnosis" (OUP) which is available at a reduced price at our practice.

Melbourne Ultrasound for Women
Level 1, 62 Lygon Street, Carlton VIC 3053
Phone: (03) 9663 3999     Fax: (03) 9663 3555
From outside Melbourne Ph: 1 800 638 810
www.melbuswomen.com.au
 

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North East Valley Division General Practice, Victoria, Australia, Disclaimer 
Level 1, Pathology Building, Repatriation Campus, A&RMC, Heidelberg West VIC 3081. .. map
Phone: 03 9496 4333, Fax: 03 9496 4349,  Email: nevdgp@nevdgp.org.au
Please note: NEVDGP does not provide an on-line consultation

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