I'm not convinced by the data on singleton pregnancies

Professor Ian Craft, London Fertility Centre

The study, involving more than 5,600 IVF children, and more than 17,000 children in total, is one of the largest ever conducted into the mental development of children conceived through fertility treatment.

Overall, children born after IVF were 3.7 times more likely to develop cerebral palsy, and four times more likely to suffer from suspected developmental delay.

Single worry

However, even single babies born after IVF were 2.8 times more likely to develop cerebral palsy.

This statistic cannot be explained so easily, although problems were more likely in single babies born early and weighing less.

Professor Ian Craft, from the London Fertility Centre, said that it was "widely appreciated" that multiple pregnancies - whether natural or via IVF, carried such risks.

However, he said that it was too early for potential IVF couples to panic about the apparent threat even to single pregnancies.

He said: "I'm not convinced by the data on singleton pregnancies. When other factors were taken into consideration, the increased risk attributable to IVF was not statistically significant."

An accompanying commentary by experts from the Department of Obstetrics and Gynaecology at Monash University in Melbourne, Australia, said that while the study was interesting, it left many questions unanswered.

"The study is valuable, but does not remove the need for clinical studies," they wrote.

Studies underway

There have not been many detailed studies examining the health of IVF children.

More recently doctors are awaiting results of studies into the long-term development of babies born following a technique called ICSI, in which a single sperm is injected into the egg.

There were some concerns that the process might increase the risk of developmental problems - at the moment it is too early to say with any confidence.

The extra risks posed by multiple pregnancies has led to pressure for steps to be taken to reduce the risk of IVF patients carrying twins or triplets.

The key factor is the number of fertilised embryos reimplanted into the womb - with many experts advocating one rather than two or three.

They say that in younger patients, this does not reduce the chances of achieving pregnancy.

However, some doctors say that in older patients, or those who have suffered repeated IVF failures, increasing the number of implanted embryos might be the only way to achieve a pregnancy - and would not necessarily increase the risk of multiple pregnancy.