IVF treatment is more than twice as likely as natural conception to provoke major birth defects in babies, a report shows today.

The freezing and thawing of embryos or because older couples usually employ IVF procedures could be the cause, say researchers.

A study showed that up to 9% of infants conceived by artificial fertilisation methods were diagnosed with major birth defects, compared with 4.2% of naturally conceived children.

The research, carried out in Australia, showed that so-called "test-tube" babies had more major cardiovascular, genitourinary, chromosomal and musculoskeletal defects.

The report, published in the New England Journal of Medicine, said major birth defects were apparent in 8.6% of infants conceived by intracytoplasmic sperm injection and 9% of babies conceived by in-vitro fertilisation.

This compared with 4.2% of naturally conceived children who suffered from birth defects.

The report's co-author, Jenny Kurinczuk, from the University of Leicester, identified a number of factors that might increase the risk of birth defects.

"These factors could include the relatively older age of infertile couples, the underlying causes of their infertility and the medications used to induce ovulation and sustain conception," she said.

"It might also be associated with the procedures themselves such as the freezing and thawing of embryos or the method of fertilisation."

Dr Kurinczuk, senior lecturer in reproductive and perinatal epidemiology at the university, said it was important that couples considering assisted conception were aware of the risks.

"While a doubling of risk is significant, it must be remembered that more than 90% of infants had no birth defects," she said.

"However this is information that couples need to have so that they can make their own assessment of the risk."