Test tube babies are more than twice as likely to be born with serious defects, according to research published yesterday in the New England Journal of Medicine.
The study found that 9% of infants conceived after IVF treatment had a "major birth defect" diagnosed before their first birthday.
This compared with a figure of 4.2% for naturally conceived children.
The birth defects ranged from life-threatening conditions such as malformation of the heart, to Down's syndrome, club foot, or cleft lip and palate.
Jenny Kurinczuk of Leicester University, co-author of the report, said that the results would be controversial because they challenged earlier studies suggesting that children conceived by IVF were no more at risk of abnormalities.
She said that she found the methodology of previous research was questionable.
Her results were based on a study of registers of births, IVF treatments, and birth defects in western Australia.
Major birth defects were found in 8.6% of babies conceived by intracytoplasmic sperm injection (ICSI), a more recently introduced technique of injecting single sperms to fertilise the egg.
This compared with 9% for babies conceived using conventional IVF, and 4.2% for those conceived naturally.
The results were adjusted to discount the fact that IVF mothers tended to be older due to the time they had spent trying to conceive naturally before seeking help.
Dr Kurinczuk said it was not yet possible to say whether the increased risk was caused by the treatments used in fertility clinics, or genetic abnormalities in the parents that might have contributed to their infertility.
Further research was needed to establish whether there were risks attached to clinic procedures, such as the freezing and thawing of embryos or the method of fertilisation.
Dr Kurinczuk, who is senior lecturer in reproductive and perinatal epidemiology at the university, said she believed it was important that couples considering assisted conception were given as much information as possible.
"While a doubling of risk is significant, it must be remembered that more than 90% of infants had no birth defects. However this is information that couples need to have so that they can make their own assessment of the risk."
