Health: Every mother-to-be's nightmare: New antenatal screening techniques are spotting more abnormalities than ever, but raising too many false alarms, says Cherrill Hicks

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Janet Carrington was four months pregnant and looking forward to the birth of her second child when a midwife knocked on the door of her home one evening last January. 'She said there was something wrong with the baby and I was to go to the hospital for further tests,' recalls Mrs Carrington. 'I was devastated. I'd had a scan quite recently and been told that everything looked perfect.'

One week earlier, Mrs Carrington, aged 35, had had a blood sample taken by her GP. Without her knowledge, the sample was used by her local hospital for the 'triple test', a relatively new method of screening pregnancies for chromosomal abnormalities which can cause Down's syndrome or spina bifida.

By measuring the levels of different chemical 'markers' in the blood and combining these with her age, the test assesses the woman's risk of having an affected baby. Women who have a 'positive' result - where the risk is 1 in 250 or greater - are then offered amniocentesis which can show for certain if the foetus is abnormal.

Mrs Carrington's test was positive. She was told her risk of having a Down's syndrome baby was 1 in 30. On the basis of age alone, her risk would have been 1 in 380.

She refused the amniocentesis: she felt she could not have faced a termination anyway. But the next six months of her pregnancy were 'absolute hell', clouded by uncertainty. 'My worst fear was that I wouldn't be able to love the baby. I kept looking at other Down's syndrome babies and wondering: could I love a baby like that?'

Her second child, Henry, was born at the beginning of June. He is perfectly normal. But his mother is bitter about what happened and easily distressed at the memory of her pregnancy. 'I feel cheated out of that six months, it was such a terrible time to go through.'

Her experience is far from rare and becoming more common as 'risk assessment' in pregnancy becomes more sophisticated, and the number of tests on offer multiply. While the new tests increase the chance of detecting an abnormal baby, they can also cause distress, especially if the woman is not offered adequate support.

In the past, the only women offered amniocentesis and other diagnostic tests for abnormality were older women thought to be at higher risk of Down's syndrome or other chromosomal abnormalities. Yet 70 per cent of all the 1,000 Down's babies delivered in Britain each year are born to women under 35.

The triple test and other forms of biochemical screening are now being offered in more than two-thirds of health authorities. While the new tests are not diagnostic, they can give a more precise estimate of risk than age alone. Offered to women of all ages, with amniocentesis an option for those with positive results, they can detect more than twice as many Down's syndrome pregnancies as the old method.

But there is growing concern about the drawbacks - notably the fact that most women whose results are 'positive' are carrying normal, healthy babies. The fear caused by a 'false positive' result does not quickly go away: the anxiety created can continue even after the birth of a healthy baby.

Jim Neilson, professor of obstetrics and gynaecology at Liverpool University, where biochemical screening was introduced last January, says he is 'very concerned' at the distress the high rate of false positives is causing to women, particularly younger ones who have not given much thought to foetal abnormality. 'They are jogging happily through their pregnancy when suddenly there is a phone call out of the blue,' he says.

'People are used to either good or bad news. Now they are presented with a figure on a spectrum of risk, which is difficult to take in.'

Kypros Nicolaides, director of the Harris Centre for Foetal Research at King's College Hospital, London, argues that labels such as 'positive' and 'negative' can be misleading. 'If your risk comes back at 1 in 249 you are positive, yet if it's 1 in 251 you are negative,' he points out. 'In reality there is no natural break in risk, just a rising curve.'

With a detection rate of only 60 per cent for the triple test there is also the problem of 'false negative' results. Women are told that their risk is low - but then give birth to an affected baby. Mary Sawtell, from the Down's Syndrome Association, says she deals daily with anxious couples caught up somewhere in the screening procedure. 'I have spoken to many couples who have had a false negative result and who feel extremely cheated and angry when a new baby is diagnosed.'

Therese Marteau, director of the psychology and genetics research group at Guy's and St Thomas' Medical School, London, is looking at whether a false negative result which is followed by the birth of a Down's syndrome baby makes it more difficult for parents to accept the child. 'Some parents to whom this has happened are talking about litigation, or adoption,' she says.

Similar anxieties can result from another antenatal screening test known as nuchal translucency scanning, which is thought to detect about 85 per cent of foetuses with chromosomal abnormalities. It can be carried out at 11 weeks. A membrane at the back of the foetal neck, known as the 'nuchal fold', shows up on ultrasound as a large black space and may be an indication of abnormality. The space is measured and the result computed with the woman's age to predict her risk of carrying an affected child. But like biochemical screening, the nuchal scan is not a diagnostic test and is an indication of risk only.

Jill Brown, pregnant for the second time at the age of 35, was told after a nuchal translucency scan that her chances of carrying an abnormal baby were 1 in 43. Her age- related risk was 1 in 220.

'I went home and said to my partner 'I think I'm going to have a Down's syndrome baby'. I tried to blank out the fact that I was pregnant and might have to have a termination. I didn't tell anyone, not even my mum. I'd been enjoying it until then.' Ms Brown had an early amniocentesis the following week; the result was negative and she gave birth to a healthy baby.

Supporters of the new screening methods point out that some anxiety is inevitable and even appropriate with the new screening methods. 'Anxiety may be the price women pay for the benefits of new knowledge,' says Nicholas Wald, professor of environmental and preventive medicine at St Bartholomew's Hospital, London. He was instrumental in developing the triple test. With adequate information and counselling, 'inappropriate' anxiety can be avoided, he argues.

But the evidence suggests that in many hospitals, women who are having the tests are not getting the right information or receiving the support they need. A study published in the British Medical Journal of 20 women who screened positive, found that some did not know they were being screened for Down's syndrome, and that women's anxieties after a positive result were often not dealt with. It also found that anxiety was a factor even for women who went on to have an amniocentesis and got a normal result. 'If the baby doesn't have Down's what does it have?' asked one shortly after delivery.

Jo Green, a researcher from the Centre for Family Research at Cambridge University and one of the authors of the study, found that more than 40 per cent of obstetricians surveyed said they did not have enough funds to provide adequate counselling for biochemical screening programmes. 'For the most part staff doing the test do not understand it,' she says.

Even supporters of the new screening methods are doubtful about the piecemeal way programmes are being rushed through in many districts. Professor Wald says antenatal screening should, like breast and cervical cancer screening, be organised as a national public health service, so that standards are the same everywhere.

Professor Nicolaides argues that a 'massive reorganisation' of antenatal services is needed to take on board the anxieties generated. 'We used to tell everyone under 37 they were at low risk so they didn't need to think about it,' he says.

'Now we have to subject that 95 per cent of unsuspecting women to the fact that they do have a risk after all. By our greater knowledge we have created anxiety and we must limit the potential for long- term damage.'