SCREENING FOR FOETAL ABNORMALITIES: NEURAL TUBE DEFECTS (NTDS)

NTDs affect the spine and/or the brain. They account for about fifty per cent of all abnormalities found at birth. They include anencephaly (gross deformity of the brain) - always fatal at or before birth - and spina bifida in which some of the vertebrae might be missing and some spinal cord tissue may protrude outside the body. The latter is called an 'open' NTD; if tissue does not protrude it is defined as 'closed'. With 'open' (i.e. severe) spina bifida, the majority of the babies die, are paralysed or are otherwise severely disabled. Less severe forms of spina bifida are not fatal, however, and it cannot be assumed that abortion is necessarily justified on the grounds of 'abnormality' alone, as many people with spina bifida will strongly argue. Of course it is also arguable whether abortion on the grounds of severe spina bifida alone is necessarily justifiable. About one baby in 1,000 is born with anencephaly in England and Wales every year, and one baby in 500 with spina bifida.

Very little is known about what causes NTDs, though there are striking class and regional variations in incidence which suggests at least some environmental influence.

The most straightforward test for NTDs is carried out on the mother's blood between sixteen and twenty weeks. This is the alpha-foetoprotein or AFP test. It was discovered in the early 1970s that women carrying babies with open NTDs had an above average level of AFP in their blood streams at around seventeen weeks. The test helps to detect a foetus with a NTD with some time for the parents to choose whether or not to abort it. A high AFP can also sometimes indicate that the baby is not growing properly in the womb. Today many health authorities offer routine AFP tests at around sixteen or seventeen weeks, and it is increasingly argued that it should be made routine throughout the country.

But should it be? This is what is currently known about the AFP blood test:

It will fail to diagnose, on average, around ten per cent of open NTDs (the most serious ones) and ninety-five per cent of closed NTDs (less serious ones).

A substantial number of positive results are false alarms. One reason for raised AFP levels may be twins. Another may be wrong dates; even a week can upset the calculation. In order to find out whether it is a false alarm, every positive AFP result is further checked, first by taking another blood sample, second by ultrasound and, if all else fails, by amniocentesis. AFP tests consequently often cause a considerable amount of worry, usually unnecessary, and considerable medical intervention, again often unnecessary. Furthermore, amniocentesis, as we shall see, is not without risk.

We believe that the stresses involved in AFP screening could be dramatically reduced. For this to happen, far more resources, including staff time, should be devoted to providing adequate counselling, information and support for women. AFP screening provides an example of how even the introduction of a simple blood test can have far-reaching effects, particularly if careful attention is not paid to the way it is given, and to its meaning generally. Ideally, no one should be offered it without a full explanation, the clear opportunity to refuse and a firm commitment from the health service to give supportive backup.

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Women's Health