Infertile women who pay thousands of pounds for an embryo screening test designed to improve the success of IVF treatment may be lowering their chances of having a baby, the largest trial of the procedure has found.

Preimplantation genetic screening (PGS), which aims to identify embryos that will not develop normally, could actually be reducing birthrates, research in the Netherlands suggests.

Other scientists, however, said that researchers with poor skills may have been inadvertently killing the embryos they were trying to test.

The PGS test, which is offered by eight clinics in Britain, is designed to detect genetic errors known as aneu-ploidies. It costs between £1,500 and £2,000, and in 2004, the last year for which figures are available, 103 women were treated in Britain.

Aneuploid embryos have either too many chromosomes or too few. These almost always either fail to implant in the womb or miscarry. The minority that survive invariably have problems such as Down’s syndrome.

PGS involves removing a cell from an IVF embryo to test its chromosomes. Only embryos with a healthy number are transferred to the womb.

The Dutch research, led by Sebastiaan Mastenbroek, of the University of Amsterdam, is the largest trial to assess it on a randomised basis.

In the study, published in The New England Journal of Medicine, Mr Mastenbroek found that, 12 weeks after embryos were implanted to the womb, 37 per cent of the group who did not have screening were pregnant, against just 25 per cent of those who had PGS.

Mr Mastenbroek told the European Society of Human Reproduction and Embryology conference in Lyons: “In theory PGS is a great technique, but the practice shows us it’s not.”

Other experts pointed out that the Dutch group had little experience of PGS. Dagan Wells, a geneticist at Yale University, said that poor procedures appeared to be killing the tested embryos before they were implanted.

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