BBC's Simon Ward on problems with an in-vitro fertilisation technique

The treatment involves injecting a single sperm cell directly into a female egg, thus virtually guaranteeing fertilisation success.

Called intracytoplasmic sperm injection, or ICSI, it was hailed as a breakthrough when introduced in 1993 and has since been widely adopted at clinics all over the world.

Widespread technique

Tens of thousands of children are thought to have been born as a result of ICSI. The technique is widespread in UK In-Vitro Fertilisation clinics, accounting for between a quarter and a third of all treatments.

But some critics have warned that little is known about the possible long-term effects on children conceived in this way.

Researchers in Australia have published findings suggesting ICSI children have a significantly greater than average chance of suffering delayed mental development at one year old.

The findings appeared to conflict with another study which found no evidence of unusually slow mental development in two-year-olds.

But experts said they served as a warning of the dangers of manipulating reproductive cells or intervening with the natural process of conception.

Researchers led by Dr Jennifer Bowen, from the Department of Neonatology, Royal North Shore Hospital, St Leonard's, New South Wales, compared 89 ICSI children with 84 children born after routine In-Vitro Fertilisation (IVF) and 80 who were conceived naturally.

'Significantly delayed development'

There were no physical differences between them, but 17% of the ICSI showed "mildly or significantly delayed development" at one year old.

This compared with just 2% for the traditional IVF children and 1% for those who were naturally conceived.

ICSI differs from normal IVF in that sperm is injected straight into the female cell.

In the traditional treatment sperm and ripe eggs are simply mixed together.

The sperm cells still have to struggle through the outer membrane of the egg cell before fertilisation can occur.

As a result, ICSI allows a defective sperm cell that would normally fail at the first hurdle access to the egg.

Some experts have warned that a defect in a sperm cell's shape or ability to move may reflect underlying abnormalities which could be transmitted to the child. It was known that some infertile men had an increased risk of genetic anomalies.

Physical disruption

It was also possible that the physical disruption of the egg cell membrane might effect the developing embryo.

Writing in the Lancet medical journal, the Australian researchers said: "Because the ICSI procedure bypasses the usual process of natural selection of sperm, selection of sperm with minor genetic abnormalities may occur that may have an influence on infant development resulting in an increased incidence of children with mild developmental delays..

"Our findings support the need for developmental follow up of children conceived by ICSI to determine whether they are at increased risk of intellectual impairment or learning difficulties at school age."

A second study by researchers in Belgium looked at 201 ICSI children and 131 IVF children.

A comparison of their mental development at two years old showed no difference between them.

More evidence needed

Professor Robert Winston is sceptical about the new research

Leading IVF pioneer Lord Robert Winston, from London's Hammersmith Hospital, was sceptical about the findings.

He said: "So often difficulties like this have been reported after new treatments are introduced, and then found not to be substantiated. It happened when IVF started. I would want to see more evidence.

"My advice to parents would be that there are certainly concerns like this which suggest some limited evidence of these sorts of problems, but they are by no means proved.

"I would imagine most couples hearing about this report would want to go ahead and have children anyway."