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At about thirty weeks of pregnancy, a second battery of diagnostic tests is usually given.

In the last ten weeks of pregnancy any maternal complications may become more serious as the foetus gets rapidly bigger and heavier. By far the most common is raised blood pressure (termed hypertension) which may be a sign of pre-eclampsia.

Moreover, by this stage in pregnancy the foetus is likely to survive if delivered early. In legal terms a foetus is viable at twenty eight weeks. In medical terms it can sometimes be earlier. If complications become too serious it is therefore possible to deliver a child either by inducing labour or by a Caesarian operation, and certainly by thirty-two weeks it has an extremely good chance of surviving.

The combination of these two facts - a higher risk of complications and a viable baby - has led to the introduction of a series of tests to see that the foetus is developing normally. They divide roughly into two groups: the biochemical, which have been in use for some time, and the biophysical, which are much newer. Most women will only encounter these if there is a suggestion that something might go wrong, but some doctors have proposed that they should be introduced more or less routinely. There is very little justification for this. Many involve considerable anxiety and stress and, as we will see, the predictive value of all such tests is questionable. A study carried out in a London hospital several years ago found that none of them was capable of predicting more than one in four complications (predictive value of only about twenty-six per cent) and that the best warning signs of a problem in pregnancy was the health of the woman herself.