Is the screening of embryos for cancer-causing genes just one more step down the slippery slope to designer babies? Sonya Dowsett meets Joy Delahanty, the geneticist who tried to produce a 'cancer-free' baby

Joy Delahanty does not look like a propagator of controversy. Yet the news last week that she had tried to produce the world's first "cancer-free" baby propelled her into a storm of debate.

Delahanty, professor of human genetics at University College London, is one of the developers of pre-implantation genetic diagnosis - in which embryos are removed from a mother's body and tested for genetic defects before being replaced to grow normally. In the case that hit the headlines, Delahanty screened the embryos of a woman with a history of severe bowel cancer for the cancer-causing gene. Having weeded out defective embryos she implanted a healthy one in the womb. Although that pregnancy failed, her team expects to replicate the treatment with other couples later this year.

Available only to parents with a family history of severe genetic disorders, this form of diagnosis is controlled by a government body that licenses its use at a handful of centres. Still, pre-implantation genetic diagnosis has many critics. Where will it end? they ask. Is it only a matter of time before embryos are screened for genes causing myopia, learning difficulties or a tendency to plumpness?

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No, Delahanty says firmly. "We're allowed to use (this technique) only for serious genetic disorders. Anything that prevents a person growing up healthy and living a normal life is classed as genetically disabling. Something that means a person will need lifelong care warrants testing if a family wants to prevent more people being born like that."

Nonetheless, she cannot deny that this latest case is a step further than scientists have gone before. Pre-implantation genetic diagnosis had never before been used to try to prevent the birth of a child at risk of cancer. Until now, doctors have screened only for diseases that affect a child from birth, such as muscular dystrophy. Sufferers from this terrible wasting disease are likely to be wheelchair-bound from birth and then die in their mid-teens.

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Now disorders that kick in during adulthood are being selected for screening. What does Delahanty say to the charge that she is in effect preventing the birth of individuals who will develop illness only late in life? "I say go talk to the families." She cites the woman with the predisposition to bowel cancer. "Not only did she have radical treatment herself, but she saw two sisters die in their twenties and as a teenager she saw her mother die, all from that severe form of bowel cancer. She does not want to bring into the world a child who will have the same problems."

The only diseases doctors can currently screen for are "one-gene disorders" or disorders linked to the baby's sex, such as haemophilia or Duchenne muscular dystrophy, which solely affect boys. Delahanty claims that traits such as intelligence or height are too complex to test for because they are governed by as many as 10 or 20 genes: "Even if we wanted to we couldn't because the genetic basis (for these traits) is not understood." With any treatment involving human embryos having to undergo strict inspection before the Human Fertilisation and Embryology Authority will grant a licence to practise, Delahanty believes that screening for anything but serious genetic disorders is unlikely ever to be allowed in the United Kingdom.

Pre-implementation genetic diagnosis was developed in the Hammersmith Hospital in London in the late 1980s by a team including Delahanty and Robert Winston, who is continuing research into the technique there. For the first few years Hammersmith was the only place in the UK licensed to practise the procedure. Last summer the HFEA granted three more licences, to University College London, St. Thomas's and Leeds.

Worldwide, 166 births using the technique have been recorded, all for the prevention of severe disease. Selection of embryos on grounds of sex will soon become available in Italy and Saudi Arabia, and some British doctors are offering this to couples prepared to travel abroad for the treatment. How does Delahanty feel about weeding out embryos because they are the wrong sex? "It doesn't seem morally worse than terminating an already growing pregnancy," she says. "In India they've been terminating established pregnancies on the basis of sex for years. In this country thousands of pregnancies are terminated every day for social reasons. Why is it so much worse to sex the embryo and give the parents a child of the sex they want?"

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Delahanty is convinced the long, drawn-out nature of the technique will prevent all but the most committed couples choosing it. The embryo has to be fertilised in vitro to allow it to be screened for disease. IVF is a stressful procedure involving the women taking hormonal drugs that imitate the menopause and then reverse it, causing multiple ovulation. "A couple will do it only if they are highly motivated," she says. "Those who choose this option have all got some reason for knowing what it's like to support a severely handicapped person."