Pregnancy is a worrying time for some, but it is important to remember that most people have healthy babies. Some women however, are more likely than average to have a child with a genetic problem, and these women may be offered a test in pregnancy. Various pre-natal procedures are available, generally being performed between 10 and 20 weeks' gestation. One of these is an amniocentesis, together with the timing, safety and accuracy of this test are important factors that must be considered.
What does an amniocentesis detect?
Amniocentesis is used most often to detect chromosome disorders, with Down' syndrome (previously called Mongolism) being the most common.
Amniocentesis can also pick up most cases of Spina Bifida (a disorder where the spine does not form properly). Most laboratories will routinely test for this, even if the amniocentesis is not done for this reason.
Some other disorders can also be detected, but the laboratory must be informed that there is a history of another genetic problem in order for additional tests to be carried out.
Who can have an amniocentesis?
Of course, the final decision about having the test is yours, but the following women may be offered amniocentesis':
1. Women 35 years of age or over (at the time of conception) - this is because the risk of having a baby with Down's syndrome increase as the mother gets older, as shown in the table below:
Mother's age and the risk of having a Down's syndrome baby:
25 years - 1 in 1250 38 years - 1 in 150
30 years - 1 in 900 40 years - 1 in 90
35 years - 1 in 300 45 years - 1 in 25
2. Women who have had a blood test indicating that there is an increased risk for having a baby with Down's syndrome or with a condition such as Spina Bifida.
3. Couples who have already had a baby with a certain problem, or have a family history of specific genetic problems ( the laboratory / a genetic counsellor must be consulted to ensure that the latter disorders can be pre-natally detected)
4. Women who have already had a previous pregnancy terminated for a genetic problem.
5. Women who have had something unusual found on ultrasound scan, which may be associated with a chromosomal or biochemical problem.
`How is amniocentesis performed?
The test involves taking a sample of the mniotic fluid ( "water" ) which surround the baby in the womd. fluid contains cells from the baby and substances from the pregnancy which can be tested in the laboratory.
Amniocentesis is an outpatient test, usually done at around 14 - 16 weeks of pregnancy (calculated from the first day of your last menstrual cycle). You do not have to be admitted to hospital or have a general anaesthetic.
An ultrasound scan is done first, to check the stage of the pregnancy and the position of the baby and the placenta (afterbirth). You will need a full bladder for this scan. The skin over the womb area is then cleansed with antiseptic solution, after which a fine needle is passed into the womb and a sample of the fluid that surrounds the baby is removed with a syringe. The sample is then sent to the laboratory.
Is the amniocentesis test painfull?
Most women say that the test is not a painful one, and feels no more painfull than having a blood test or an injection. Generally, women agree that the thought of it is worse than the actual test.
What happens after the procedure?
The test may take up to 25 - 35 minutes in total ( including the ultrasound scan). Afterwards, it is suggested that you go home and rest for the remainder of the day. It is also a good idea to take things easy for a few days thereafter, and to avoid any heavy lifting or strenuous exercise. Sometimeswomen get a "tightening" feeling in the womb afterwards /or feel a little sore the next day. This is not unusual, but if you are worried about anything, do contact you doctor.
When do you get the results?
The chromosome test (mainly for Down's syndrome) involves "growing " the cells in the laboratory, and the results for this usually take 3 - 4 weeks. The Spina Bifida result is faster, and only takes about 2 days. Your doctor will be sent the results. In the event of a problem being found, your doctor will be telephonically contacted and you will be offered the opportunity of discussing the findings with a genetic counsellor. Very occasionally, the cells do not grow well in the laboratory and the amniocentesis will have to be repeated in order to obtain results. If this happens, your doctor will be contacted immediately and you will be offered a repeat amniocentesis. Therfor, if you are planning to be away within 2-3 weeks of having the amniocentesis, please inform your doctor.
What if the results are abnormal?
If the results show anything abnormal, this will be discussed with you and you will be told how the abnormality is expected to affect your baby. You and your partner can then consider whether or not you want to continue with the pregnancy and you will be given a chance to fully discuss the findings before making any final decision. If you decide that you want to have the pregnancy terminated, this can usually be arranged fairly quickly and is legal under the South African Abortion and Steriliztion act.
What are the risks of an amniocentesis?
Amniocentesis is a well-established and widely available method for pre-natal diagnosis. However, it is known that the test can sometimes cause a miscarriage. It is estimated that 1 in 150 women who have the test will miscarry as a result of the procedure. (The skill of the practitioner will to a large extend determine the risk of miscarriage). Most of these miscarriages will occur within 2 weeks of having the test. Other than this, it is believed that in general, emniocentesis is NOT harmfull to you or your baby.
Is the test reliable?
The chromosome part of the test is very accurate. The Spina Bifida test on the fluid is not quite as accurate, but does pick up a great majority of pregnancies with an opening in the spinal column.
If you are having an amniocentesis for other genetic reasons, you should discuss the accuracy of the test, in you particular case, with a genetic counsellor or the doctor in charge of the genetics laboratory.
It is not possible to write everything down here, and each person may have their own particular questions and points they wish to discuss. Therefore, please ensure that your doctor or a genetic counsellor answers your questions before you decide whether you would like the test performed.
(Information supplied By Jennifer Rosendorff PhD, Cytogenetics Laboratory, PO Box 8475, Johannesburg 2000, Tel: 011-679-5850)
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