Birth defects caused by in-vitro fertilisation are up to twice as common as previously thought, a large-scale study has found.

Doctors at fertility clinics should routinely warn patients of the dangers, say researchers.

Scientists carried out a survey of 33 French centres collecting data on more than 15,000 births from 2003 to 2007.

The study, the largest of its kind, found evidence of a higher than expected rate of serious congenital abnormalities.

Research leader Geraldine Viot, from the Maternite Port Royal Hospital in Paris, said: "We found a major congenital malformation in 4.24% of the children, compared with the 2-3% that we had expected from previous published studies.

"This higher rate was due in part to an excess of heart diseases and malformations of the uro-genital system. This was much more common in boys.

"Among the minor malformations, we found a five times higher rate of angioma, benign tumours made up of small blood vessels on or near the surface of the skin. These occurred more than twice as frequently in girls than in boys."

Dr Viot presented the results at the annual meeting of the European Society of Human Genetics in Gothenburg, Sweden.

She said couples considering assisted conception should be informed about the risks without having to ask.

Most fertility doctors only told patients about the risk of birth defects if they were asked specific questions, she claimed.

Some individual studies have reported a malformation rate of 11%, but their results are dubious.

"Given that our study is the largest to date, we think that our data are more likely to be statistically representative of the true picture," said Dr Viot.

She plans to follow up the research by investigating a further 4,000 children born in 2008, and to look at the development of IVF children who are now seven years of age.

"By following all these children, we hope to understand more about not only what can go wrong after ART (assisted reproductive technology), but why it goes wrong," she said.

"At a time when infertility is increasing and more couples need to use ART to conceive, it is vitally important that we find out as much as we can about what is causing malformations in these children, not only so that we can try to counteract the problem, but also in order for health services to be able to plan for their future needs."

Many of the defects are thought to be linked to "imprinting", the process by which certain genes are switched off or kept active according to which parent they are inherited from.

One known imprinting disorder, Beckwith Widemann syndrome, was six times more common in the IVF children studied than in the general population.

Why these problems occur is still a mystery. There could be a wide range of explanations, including infertility itself, ovarian stimulation, the maturing of eggs in the laboratory or the ICSI (intra-cytoplasmic sperm injection) technique which injects a single sperm into the egg, said Dr Viot.

She added: "We just don't know at present. Finding this out will be a major step towards improving the health of children born after ART."

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