What we have to remember is that more than 90% of babies are free of such major defects

Dr Jennifer Kurinczuk, study author

Women are given drugs to mature an increased number of eggs, then these are removed and combined with semen in a special solution in the laboratory.

Any resulting embryos are examined, and a small number reimplanted in the woman.

Heart defects

It is already believed that IVF babies are more likely to be born prematurely, putting them at slightly higher risk of developmental problems.

The research is published in the New England Journal of Medicine, and was carried out by linking registers of IVF children with those listing children born with significant defects.

These included congenital heart problems, Down's syndrome, club foot and cleft lip and palate.

Being born prematurely would not explain these defects.

In total, 837 babies born after IVF were included, and 301 babies born after a more recently-introduced technique called intracytoplasmic sperm injection (ICSI), in which a single sperm is injected into the egg.

These were compared with 4,000 babies conceived naturally.

Older women

Overall, women undergoing fertility treatments had an 8.6% chance of giving birth to a baby with a serious defect.

The expected rate among naturally conceived babies is roughly half this.

IVF involves fertilising eggs in a test tube

In general, IVF mothers tend to be older than non-IVF mothers, mainly because couples spend years trying to conceive before seeking help.

This might be expected to increase the rate of birth defects - for example, Down's syndrome is more common in babies born to older mothers.

However, the latest study was adjusted to take account of this, and the results were not swayed by the age gap.

Reassuring

There was no apparent difference in risk between IVF and ICSI.

Dr Jennifer Kurinczuk, one of the lead authors of the study, from the University of Leicester, said that her results should be taken alongside other less alarming studies.

"It is neither the first or the last word on this issue - although we believe our study is robust," she said.

She said that the study was not designed to look for reasons why babies born after fertility treatment appeared more vulnerable to such defects, and called for further research into the issue.

It was possible, she said, that genetic abnormalities in the parent - perhaps linked to their own infertility - were responsible.

But she could not rule out the possibility that some stage of the IVF process may be to blame.

However, she said: "What we have to remember is that more than 90% of babies are free of such major defects."

"Parents just need to be fully informed of the risks before they undergo such treatment."

London specialist Mohammed Taranissi said that risks were likely to be different for every different couple embarking on treatment.

He told the BBC: "The advice is to make individual assessments of the risk to each particular couple, rather than make general rules from the vast majority."

A spokesman for the pressure group Life, which opposes IVF treatments, said: "Over half of women seeking IVF treatment are registered as suffering from an unknown cause of infertility.

"We should be diagnosing the infertility and seeking treatment, not concentrating on a costly, artificial procedure that has at least an 80 percent failure rate and produces an increased risk of abnormalities."