8 January 2020

What is amniocentesis?

amniocentesis

The term amniocentesis is derived the Latin amnion, the membrane that forms the protective sac around a growing embryo, and the Greek kentesis meaning to puncture or to prick. This accurately describes the amniocentesis definition of the insertion of a needle through the amnion in order to extract a small amount of amniotic fluid. This fluid can reveal genetic information about the foetus, including the presence or absence of genetic abnormalities.

The amniocentesis test has been used for the latter purpose since the 1950s and for a long time it remained the major diagnostic test for abnormalities such as Down’s Syndrome. The test has a high accuracy rate of around 99.4%. It is an invasive test and carries a small but highly significant risk; one which can lead to some extremely difficult choices for prospective parents. This is the one-in-a-three hundred chance that the test will cause a miscarriage.

What happens during an amniocentesis test?

Firstly, an ultrasound to determine the position of the foetus is carried out so that the needle can be inserted as far away as possible while still being able to extract some amniotic fluid. Then, a small area of the abdomen is cleaned with sterile solution and local anaesthetic may be applied. Using the ultrasound guidance, the doctor inserts the needle through the abdomen, through the amniotic sac and into the amniotic fluid. A small amount of fluid is drawn up through the needle. The patient can then return home after receiving the instructions to minimize the risks after the procedure.

She may have slight cramping similar to period pains and is advised to abstain from sex or any strenuous activity for a day or two. She can expect to receive the results within three days if only Down’s Syndrome, Edwards’ Syndrome and Patau’s Syndrome are being tested for, but if other rarer conditions are also the subject of testing, results can take up to three weeks.

Who is the amniocentesis test for?

This prenatal test is not routinely offered to all pregnant women because of the small risk that it carries for both mother and baby. It is offered to those who are considered to be at higher than normal risk of a genetic disease. This includes people who:

  • Have had a previous pregnancy or birth with a birth defect;
  • Have had screening or ultrasound scans which show the possibility of a genetic disease;
  • Have a genetic condition in their family history;
  • Are over a certain age, since the likelihood of defects increases for older mothers.

A negative test result can provide reassurance that the conditions tested for are absent. This is not a complete guarantee that there are no problems as there are some conditions it does not detect or appear later, but it is 99% reliable for identifying cases of sickle cell disease, cystic fibrosis and muscular dystrophy as well those already mentioned. In addition, because the test must be carried out in conjunction with an ultrasound scan, the scan itself could detect conditions which the amniocentesis test does not, for example cleft palate, club foot or heart defects.

What are the risks of an amniocentesis test?

The major risk is that the test will cause a miscarriage. It may be only 1% but, understandably, for many women it is not a chance they are willing to take. For others, the decision to have an amniocentesis test is only reached after a difficult and distressing process of weighing up the potential benefits against the potential hazard. In the small number of cases where the test does cause a miscarriage, it is likely to occur within two or three days, but in some cases it can occur up to two weeks later. There is no evidence that a woman can do anything during this time to reduce the risk but to follow the instructions given after the procedure.

There are other smaller risks associated with the test, for example the slight risk of infection associated with any invasive medical procedure. However, the rate of severe infection as a result of the amniocentesis test is less that one in a thousand. It is also possible, if a woman’s blood group is rhesus negative and the baby’s is rhesus positive, for sensitisation to occur as a result of the baby’s blood entering the mother’s bloodstream which then produces antibodies to attack it. This risk is mitigated by a routine blood test in advance, and, where necessary, a preventative medication being given.

Is there an alternative to amniocentesis?

The really good news is that there is now an alternative to the amniocentesis test: non-invasive prenatal testing. As the name indicates, this is a method of testing the genetic characteristics of the embryo without any invasive surgical procedure and thus the risk of causing a miscarriage is zero.

Here’s how it works. A sample of the mother’s blood is taken in the normal way as for any other blood test. Because a small amount of the baby’s DNA is in circulation in the mother’s bloodstream, an analysis of the blood sample can identify which is the foetal DNA and which is the mother’s. As a result, the same sort of information about genetic makeup can be extracted as the information that results from testing the amniotic fluid. Non-invasive prenatal testing can identify:

  • Down’s, Edwards’ and Patau’s Syndromes and
  • The most common anomalies generated by the X and Y sex chromosomes.

There are also more advanced methods for testing the DNA present in the maternal bloodstream. These can detect abnormalities in other chromosomes, as well as detecting fragmentary losses in certain chromosomes which relate to important genetic syndromes. These extended tests would be medically recommended when abnormalities have been detected by an ultrasound scan.

In this way, pregnant women who in the past had some really tough decisions to make are now able to have their concerns allayed with no more than a blood test.

Want to find out more?

If you would like to know more about non-invasive prenatal testing or any other of our tests, techniques and treatments, do browse our website which is always up to date with the latest news about medical and technical advances in our field. Or, if you would like to get in touch with any question or concerns, contact us through our online form. Sometimes peace of mind is just a short click away!

 

 

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