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Inconsistencies in tests for foetal abnormality

By David Mullins

The tragedy that is currently unfolding around the termination of a perfectly healthy baby who was misdiagnosed as having Trisomy 18, Edward’s Syndrome, is heart breaking. It bears stark witness to the irresponsible and dangerous sloganeering engaged in by those who campaigned for ‘Safe, Legal and Rare’ abortion without ever having properly interrogated the actual risks.

From what we know so far, the parents of the unborn child were told of the ‘diagnosis’ following a procedure known as chorionic villus sampling (CVS).  This involves cells being taken from the placenta (the organ that links the mother’s blood supply with her unborn baby’s) and subsequently being tested for genetic (inherited) defects.

In the last few days since the story broke, we have been repeatedly told that this test, based on a sample is regarded as 99 per cent accurate, with a false positive rate of about 0.15 per cent. There is no doubt that this point will be continually emphasised until we are once again confirmed in the false belief that abortion is essentially a ‘safe procedure.’ The false positive rate might be correct but there are other serious risks associated to this test.

What you may not hear is the fact that chorionic villus sampling itself carries a risk of induced miscarriage at a rate significantly above the 0.15 false positive statistic. The UK’s NHS [1] says “The risk of miscarriage after CVS is estimated to be about 0.5 to 1%. This means that 1 in every 100 to 200 women will have a miscarriage after having CVS.”

The HSE [2] on the other hand put the risk of miscarriage after CVS at 2.7 per cent, which is about 1 every 37 women. “One 16 year study found that the risk of miscarriage after CVS was 2.7 per cent, but in the control group of women, who did not have CVS, the risk of having a miscarriage was 3.3 per cent. This suggests that CVS does not significantly increase your risk of having a miscarriage.”

The HSE also claims that one of the biggest influences on the risk of miscarriage could be how many procedures a particular medical centre has carried out. More experienced centres will have a reduced rate of miscarriages.

What you also may not hear is that although the probabilities of identification of ‘abnormalities’ are high, this test does not measure the severity of these disorders. Further, according to the HSE’s own information on this procedure, estimates suggest that CVS is slightly more dangerous than amniocentesis. This is to say nothing of the fact that it is possible for the procedure to fail to extract a sample of chorionic villi that can be used for testing.

What is specifically concerning here is the direct conflict in information about the extracting failure rate as provided by both the Coombe Hospital and the HSE. According to the Coombe, [3] it happens “rarely-less than 1%,” of the time; while for the HSE [2], the rate of failure to extract a sample can happen “in between 2.5-4.8 per cent of procedures.” That is a startling level of statistical variation that must be explained. This is important because the greater the failure rate, the greater the necessity for repeat procedures which in turn increase likelihood of induced miscarriage.

If you are an unborn twin the danger rate increases. According to 2010 guidelines issued by The Royal College of Obstetricians and Gynaecologists’ “the miscarriage rate after CVS and amniocentesis is likely to be higher in multiple pregnancies as compared with singletons.” The RCOG [4] put the miscarriage rate at 1 in 56 pregnancies. The HSE also confirms that with respect to the CVS procedure there have been some cases of limb abnormalities in the foetus, such as missing or short fingers and toes. They go on to ‘assure’ us however that although some clinical trials have made an association between these abnormalities and having CVS earlier than 10 weeks into your pregnancy, “CVS is no longer performed earlier than 10 weeks in the pregnancy to avoid this possible complication.”

What all of this should do is spur us on to a detailed examination of the ethics of prenatal genetic testing.   Whether it will actually do this is anyone’s guess.