Health

How does testing for foetal abnormality work?

Termination may be offered where there is a condition likely to lead to the death of the foetus either before, or within 28 days of, birth

Maternity hospitals have protocols to follow when parents-to-be request testing for foetal abnormality. Photograph: iStock

Dr Muiris Houston's face
Dr Muiris Houston
Fri May 17 2019 - 20:34

Foetal abnormalities occur in 2-3 per cent of pregnancies. Under the 2018 Health (Regulation of Termination of Pregnancy) Act, termination may be offered where two medical practitioners are of the opinion formed in good faith that there is a condition affecting the foetus that is likely to lead to the death of the foetus either before, or within 28 days of, birth.

Maternity hospitals have protocols to follow when parents-to-be request testing for foetal abnormality. The major units have specialist consultants in foetal medicine, to whom patients are referred by other obstetricians. They are experts in counselling, testing for and managing all aspects of foetal wellbeing.

What is a fatal foetal abnormality?

A congenital condition affecting the foetus that is likely to lead to death before or within 28 days of birth because of a problem with its development. For example, a missing or an extra chromosome, causes many changes in the way an unborn baby develops. Edwards’ syndrome (trisomy 18) is a rare but serious genetic condition that causes a wide range of severe medical problems and causes death of the baby either before or shortly after being born.

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What is the first signal something is wrong with the foetus?

About 20 per cent of pregnancies end in miscarriage; bleeding early in pregnancy may be a signal that all is not well with the foetus. But sometimes it is a routine scan that first suggests all is not well, which may be confirmed with more specific testing.

How do you test for foetal abnormality?

It is now possible to test for foetal wellbeing using a sample of the mother’s blood. Called non-invasive pre-natal testing (NIPT), tests such as “Harmony” and “Panorama” rely on the fact that from early in the pregnancy, foetal blood cells enter the mother’s bloodstream. These blood cells are analysed genetically and can identify genetic and chromosomal abnormalities in the baby. Test results are available in about a week. The results are reported as a percentage likelihood of a foetal abnormality being present. So the NIPT, like so many medical tests, cannot be reported to the parents as an absolutely positive or an absolutely negative result.

What happens next?

If the NIPT raises the possibility of a foetal abnormality, a second, invasive test is recommended. The particular test to be performed depends on the gestational age of the foetus. Chorionic villus sampling (CVS) is performed early in pregnancy ( between week 11 and 13). In CVS, a sample of cells is taken from the placenta (the organ that links the mother’s blood supply with her unborn baby’s). For amniocentesis, the fluid from around the baby – amniotic fluid – is examined. It is usually carried out between the 15th and 18th week of pregnancy.

CVS is estimated to give a definitive result in about 99 per cent of cases. However, it cannot test for every birth defect and like all medical tests, it’s not always possible to get a conclusive result. Depending on the condition being tested for it can take between five days and three weeks for a result to come back. For many women who have CVS, the results of the procedure will be “normal”. However, a normal result doesn’t guarantee that her baby will be completely healthy, as the test only checks for conditions caused by faulty genes, and it can’t exclude every possible condition.

Why is a two-stage testing process employed?

Because no single medical test, whether a simple blood test or the most complex scan, is 100 per cent accurate. Every investigation in medicine throws up both false positives and false negatives. A false positive in foetal testing would mean the test indicated the baby had an abnormality when they are, in fact, developing normally. A false negative test offers false reassurance; parents could be told their baby is normal when in reality there is an abnormality present. By combining two different types of test, you reduce (but do not completely eliminate) false positive and false negative results.