Experts have warned research which found IVF babies have a higher risk of a rare eye cancer should be treated with caution.

A Dutch study suggested IVF conception could be linked with retinoblastoma, a malignant tumour of the retina which occurs in childhood.

But critics have said the study, published in The Lancet, is very small, and that larger studies would be needed to confirm the findings.

Whether treatment with ovulation-inducing drugs increases the risk of childhood cancer is an important matter Dr Annette Moll, Vrije Universeit Medical Centre

Previous research has not suggested an increased risk of the cancer amongst IVF children.

Retinoblastoma is rare, occurring in around one in 17,000 births in Western countries.

Incidence

The Dutch research was carried out after the cancer was diagnosed in five children born after IVF, within 15 months of each other.

This researchers then compared these cases with the expected incidence of retinoblastoma in the normal population.

They estimated that children conceived by IVF could be between five and seven times more likely than non-IVF children to develop retinoblastoma.

Between 1% and 1.5% of Dutch babies are born through IVF, which is used to treat 3,000 women in the country each year.

It is small and it bases several figures on assumptions rather than real observations Professor Christina Bergh, Sahlgrenska University Hospital, Gothenburg, Sweden

Dr Annette Moll, of the Vrije Universeit Medical Centre, said: "Whether treatment with ovulation-inducing drugs increases the risk of childhood cancer is an important matter, especially with the rising numbers of women undergoing treatment for subfertility.

"Future investigators should consider the number of IVF treatments, other fertility drugs given before IVF, and the possibility that serious disorders in children conceived by IVF are diagnosed earlier than those in other children who do not have such close medical surveillance.

"Our finding requires further research to confirm the association and to explore a possible causal mechanism."

'Isolated cases'

But fertility experts said the study looked at a very small number of cases.

The British Fertility Society (BFS) said it understood the study could cause concern, and that it would continue to review such reports carefully.

It added that the study admitted it had looked at isolated cases, and there was no confirmed evidence linking IVF treatment and retinoblastoma.

Dr Alison Murdoch, chair of the BFS, said: "The number of children born after IVF will continue to increase as the treatment and its availability improves.

"Sadly, it is inevitable that some of these children will develop the same problems suffered by other children who were conceived naturally.

"Whilst it is vital that we are aware of the potential risks of IVF, patients who have children as a result of IVF can be reassured that this report is an isolated finding.

"Parents should remain confident that their children need no tests for this condition."

Professor Christina Bergh, from Sahlgrenska University Hospital in G�tenburg, Sweden, whose team has been reviewing the health of children born by IVF techniques, said she had several concerns about the study.

"It is small and it bases several figures - including the percentage of IVF births in the Netherlands - on assumptions rather than real observations.

"The five children identified with retinoblastoma in the study have all been successfully treated, yet the expected numbers of retinoblastomas were based on mortality rates rather than on cases recorded in the cancer registry, which would have been more appropriate since the register includes both fatal and non-fatal cases."

She said three large studies, covering almost 20,000 IVF children, had found no increased cancer risk, and that there were no cases of retinoblastoma amongst 6,000 IVF children on the Scandinavian IVF database.

The Dutch research is published shortly after another study suggested IVF babies were more likely to suffer from Beckwith-Wiedemann syndrome, a rare gene disorder which causes too much growth, kidney abnormalities and an increased chance of tumours