Dr Howard Lee describes what it is, who is offered it and what it does

Amniocentesis is a specialised diagnostic test that involves removing a small amount – usually about 2 tablespoons (20 mls) – of amniotic fluid (liquor) from the sac that surrounds your baby in the uterus (womb). This is so that the fluid, which contains hundreds of living cells from your baby’s skin, can be examined in a laboratory for changes. These can indicate – with a great degree of certainty – whether or not there are any serious abnormalities present in your baby.

What kinds of abnormalities can it detect and what are the reasons for the test?

• To diagnose any inherited disorders that your baby may be at risk of
• To count the number of chromosomes, to see if parts of them are missing. These chromosome tests can detect disorders such as Down’s syndrome.
• To decide on the ‘maturity’ of your baby’s lungs – if a Caesarean section is being considered; for instance, in cases of foetal distress and where early delivery is desirable.
• To check for the level of AFP (alpha-feto protein) – a chemical made by your baby’s liver. This is found in very high levels in the amniotic fluid if there are certain birth defects that cause a break in the skin – such as occurs in Spina-bifida, where the vertebrae of the back fail to close up, and Anencephaly, a condition in which the brain is incomplete or missing.
• To remove excessive amniotic fluid, or look for infection in the amniotic cavity.
• Your baby’s sex can be determined, and this is important when identifying gender-linked hereditary disorders.
• Can also reveal whether your baby is Rh-negative, but this test is usually performed later on in pregnancy.

When are women offered the test and why?

The test can be performed as early as 12 weeks or as late as 20 weeks, but it has been shown to carry a higher risk of miscarriage if performed earlier than 14 weeks gestation.

• It is common practice to offer you the test if you are over 35 years old. This is because you are at a higher risk of having a baby with the chromosomal disorder responsible for causing Down’s syndrome.
• If an ultrasound examination has detected an abnormality associated with a genetic disorder, or if you have a family history that indicates that there is a risk of you having a baby with such a disorder, you will be offered the test at about 14–16 weeks gestation.
• It may be that an AFP blood test has indicated a risk of Down’s syndrome – again, the test will be offered at this time, as there is enough amniotic fluid surrounding your baby to make it easier for the doctor to take a sample without putting your baby at risk.
• If, for some reason, your baby needs to be delivered several weeks/months before your due date, it might be suggested that an amniocentesis is performed in order to check that your baby’s lungs are properly developed.

Amniocentesis is usually performed as an outpatient procedure. You will be offered either a local anaesthetic – administered by injection in the skin, or a topical anaesthetic – which can be rubbed into or sprayed onto your skin. It isn’t always necessary to have a local anaesthetic for this procedure, but you will be given the choice. A long, thin, hollow needle is then inserted through your abdominal wall – under ultrasound guidance – into the sac of fluid that surrounds your baby. The ultrasound enables the doctor, who is performing the test, to identify the safest entry point for the needle, so that your baby or the placenta does not suffer any damage when fluid is removed (it has to be done in a clear area of the amniotic sac). It is this part of the procedure which takes up most of the time (anything up to 20-25 minutes), with just a couple of minutes to withdraw the fluid.

Is the test painful?

Everyone experiences the procedure differently. Some women report that they feel it with one pregnancy and not with another. If an anaesthetic is used, you may only feel a sharp, stinging sensation for a few seconds, and a sharp pain as the needle enters the amniotic sac. Some women just report a feeling of ‘pressure’ in the abdomen, others report that it was ‘no more than having a blood sample taken’.

What risks and disadvantages are associated with the test?

Amniocentesis, while certainly being the safest of the ‘invasive’ tests, can still lead to a spontaneous miscarriage in 0.5-1% (that’s 1 in 200) of women.

About 1 in 200 women undergoing the test develop an infection or some other complication as a result – this too, can lead to a spontaneous miscarriage. It is possible to have leaking of amniotic fluid – or even a small amount of blood – for a few days after the test, though this stops after a period of bed rest. Some women can develop a high temperature, abdominal tenderness and some uterine contractions, which can also mean that there is infection.

Will I get the results straight away?

Unfortunately, no – as the sample of fluid has to be sent away for laboratory processing and culture. The waiting period can be very long – it can take up to three weeks in fact, for the results, and a diagnosis to be made. Amniocentesis is almost 100% reliable in detecting various abnormalities, but no test is perfect. Occasionally, there can be a failure – when the cells do not grow, for instance – and you may be asked to undergo another one.

If you have undergone the test, and it is found ‘positive’ for some abnormality, you then have to make a decision about a possible termination of pregnancy. At around 20 weeks gestation, it will be necessary for you to have an induced labour and birth.

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 counsellor, 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, counsellors 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 (not yet available in all areas) – 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’, was then developed for similar abnormalities. This was followed by the Triple test, an extension of the AFP test, but much more effective at determining risk. Now a Quadruple test, relatively new, and certainly one of the most accurate, has been developed for determining Down’s syndrome ‘risk’. This is not yet available in all areas. So, watch this space for further developments.

Choosing to have an invasive diagnostic test is difficult – the early weeks of excitement about being pregnant are spoiled, very suddenly, by the worry over whether to have a test that could put your baby at risk. You have to balance this risk against the risk of miscarriage and what you would do if the test detected some form of abnormality. Never be rushed into having it and take your time thinking through all aspects of the test.