Thousands of infertile women who undergo IVF treatment are risking themselves and their embryos because they are receiving too many strong hormonal drugs, new research reveals today.

More than 10,000 children - around 1.5 per cent of all live births - are born in the UK each year using the treatments. For years, clinics have chosen to place several embryos in the womb to give the best possible chance of a pregnancy, often leading to multiple pregnancies that are dangerous in themselves because they can lead to premature delivery.

Now two studies discussed at a conference in London last week show that women who receive high doses of drugs to stimulate their ovaries into producing lots of eggs - so that the best possible ones can be picked once the egg has been fertilised by sperm in the laboratory - are more likely to produce embryos with genetic defects and suffer harmful changes to their womb lining.

Most embryos will never develop into babies because the defects make it impossible for them to survive when they are implanted back into the womb. But the discovery explains why so many fertility treatments fail, with thousands of women going through several expensive and painful cycles of treatment in the hope of having a child. It will add to worries that some genetic changes may occur in the children which are not yet being picked up by doctors.

At last Friday's conference, Professor Antonio Pellicer, of the University of Valencia, presented initial results from a trial of 10 women who were put through a stimulated IVF cycle employing routine amounts of stimulating drugs, known as gonadotrophins. The same group then received half the dose in a second stimulated cycle.

Researchers looked at what happened during the time when the embryo might be implanted back into the womb, and found a high rate of abnormalities in the genes that would normally become active. When they halved the dose of the drugs, they found the rate of abnormalities fell from 50 to 33 per cent.

'It is time for a complete rethink of how we approach IVF today,' Pellicer said. 'We should be looking to reduce the dosage of these drugs and take a much gentler approach that could be beneficial to both the women and their babies.'

In another study, soon to be published, researchers in the Netherlands looked at 400 couples, and compared those who had a high dose of stimulating drugs with a group who underwent a more 'natural' form of fertility treatment now being pioneered by some clinics. Over one year, both groups had exactly the same proportion of live births, showing that the outcomes were just as good, though in the lower-dose group only one embryo was ever transferred back into the womb. That compared with more two or three embryos transferred in the other group.

In Britain, fertility clinics generally allow the transfer of one or two embryos, but some specialists feel that just one should be implanted. Problems can also arise because the ovaries are sensitive, and the hormones used can cause ovarian hyperstimulation syndrome, a painful condition affecting up to 6 per cent of patients. In August a woman died of the syndrome at Leicester Royal Infirmary.

One in six UK couples has difficulty conceiving and the problems are growing as men and women are waiting until they are older before trying for a child.