Amniocentesis – an invasive test

by Dr Howard Lee

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Will I get the results straightaway?

Unfortunately, no - as the sample of fluid has to be sent away for laboratory processing and culture. The waiting period for the results can take two to three weeks, in fact. Amniocentesis is almost 100 per cent reliable in detecting various abnormalities, but no test is perfect.

Occasionally, there is a failure - when the cells do not grow, for instance - and you may be asked to undergo another one.

Should I have an amniocentesis, and what do I need to consider?

Only you can make this decision, of course, but choosing whether to have a diagnostic test can be very difficult. You have to consider why you need the test - usually because your baby has a high risk of being affected by an abnormality.

Some couples want to know as much as possible about their baby before birth - others would rather wait until their baby is born.

If you know in advance that your baby has a birth defect, it can give you time to learn about the kind of help that will be needed. As a couple, you'll have to decide just how important it is to know whether your baby is affected by an abnormality and compare that against the risk - albeit small - of perhaps miscarrying a healthy baby, due to the test itself.

Whatever you decide, emotions run high

Although you will have to make the final decision on your own, counselling will usually always be offered. This may be from a genetic screening counselor, or from another specially trained member of the medical team, who can help you consider all the pros and cons and, whether or not you would wish to continue with the pregnancy, if an abnormality were to be detected.

Remember, counselors are not there to persuade you in any way, they are there to help you find the right answer for your particular situation. As a couple, if your baby is affected by a disability you'll have to consider the impact this will have on your lifestyle, your relationship and the rest of the family.

Are there any alternative types of investigation?

The Nuchal translucency scan - usually performed between 11-14 weeks - is the latest and most accurate screening test for Down's syndrome. It is considered to give a very accurate estimation of risk if you are over 35 years of age - but less so if you are under.

Chorionic villus sampling (CVS) - usually carried out between 11-13 weeks - can give the same information as Amniocentesis. However, it cannot detect neural tube defects like anencephaly and spina bifida, and it does carry a higher risk of miscarriage in some hospitals.

It does seem that, at the moment, the only way to know for sure whether or not your baby has any major abnormality is by amniocentesis testing.

More and more non-invasive tests are being developed using blood samples

The AFP (Alpha-Fetoprotein) test to indicate Down's syndrome and neural tube defects has been available for a long time. A Double test, measuring two 'indices', AFP and hCG was then developed, and quickly became the Triple test, which measures AFP plus beta-hCG and unconjugated estriol.

Next came the Quadruple test, which added in a check for levels of the hormone inhibin A, while the latest, the Penta Screen, assesses these four substances and also measures Invasive Trophoblast Antigen. This is relatively new, and certainly one of the most accurate screening tests to Down's syndrome 'risk' but is not yet available in all areas.

Results from all these tests are used together with maternal age, gestational age of the fetus, whether or not there are twins or more, to determine overall risk.