Cardiotocography or CTG is a method of measuring the reaction of a foetus's heart to movement, either its own movement or movements created by the very minor contractions of the womb that go on through late pregnancy. The theory is that in a healthy baby, the heart rate will speed up in response to movements and contractions, whereas the heart of a sick foetus will remain unchanged. The method relies on a machine which uses ultrasound to detect foetal heartbeats or picks up electrical activity from the heart (ECG) and then prints these out as a heart trace on a piece of paper or, occasionally, on a TV screen.

These traces can be extremely hard to interpret. One doctor maintains that: 'There are certainly trace patterns which do seem to be associated with an adverse outcome and there are some which are not. But there is a very large grey zone.' No change in heart rate, for instance, may not necessarily mean a sick foetus.

In 1978 an editorial in The Lancet worried that 'ante-natal foetal heart monitoring is bedevilled with false positives' (false alarms), and pointed out that to date there had never been a proper assessment of its value as a measure of foetal wellbeing. However, it is used routinely in some hospitals.

As with many of tests already discussed, misuse can potentially be very serious. The American magazine Hospital Practice has argued that mistimed caesarian deliveries are 'threatening to become the leading cause in newly born babies of a severe problem called "Respiratory Distress Syndrome" ', which results from a baby being born before its lungs have matured. It goes on to suggest that one of the main causes of this could be the 'widespread use of fetal heart rate monitoring in community hospitals'. The Lancet has also suggested that thousands of women and babies have suffered ill-timed and unnecessary inductions and caesarian operations because of an apparently abnormal cardiotocograph.

Attempts to check the value of CTG get caught in a Catch 22 situation. Suppose a doctor gets what she or he believes is a warning trace and decides to deliver the baby early. If it is normal then is this because it was normal anyway? Alternatively, if the baby is damaged, is this because the operation was performed too late or too soon?

One of the major problems with all measures of foetal wellbeing is that it is extremely difficult to judge when to take action and, perhaps, induce birth and when simply to watch and wait. Even if there were extremely accurate measures of foetal wellbeing in late pregnancy, which there are not, none would tell you how long a baby might continue to survive undelivered. The temptation has been to deliver earlier rather than later in an overzealous attempt to head off trouble.

If you are offered an induction or caesarian delivery on the basis of the results of a foetal monitoring test, try to make sure that the results have been carefully cross-checked with other measures of foetal wellbeing and by a full clinical check-up.

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Women's Health