Some sperm are healthier than others

The latest study, published in the Lancet medical journal on Friday, will provide some reassurance to parents of ICSI babies.

However, other fertility experts warn that much larger groups of children must be tested over a much longer period before a clean bill of health is given to the technique.

ICSI worries doctors because it bypasses some of the stages of the normal fertilisation process which may prevent genetically-suspect sperm from fertilising an egg.

In normal IVF, as many as 30,000 sperm "compete" to fertilise the egg, while in ICSI, a laboratory worker will select a single sperm.

Hidden defects

That sperm might carry genetic defects which, in the real world, would have prevented it fertilising the egg, but the worker has no way of telling, so defective genes may be passed on.

In addition, there are some fears that the injection process itself risks subtly damaging the genetic structure of egg or sperm.

However, all the studies so far suggest that there is no evidence of serious neurodevelopmental problems stemming from ICSI.

There is a slightly increased risk in some cases of a condition called hypospadias, a mild deformity of the penis, but this is likely to be due to the fact that naturally sub-fertile men are more likely to carry the gene for this condition.

Equal scores

The study examined 208 babies - aged on average 17 months, of whom half were ICSI babies, and checked their mental development.

The ICSI babies scored just as well as normally-conceived babies.

We have to understand that ICSI is still an experimental procedure

Professor Peter Barratt

The only difference was a higher rate of caesarean section among the ICSI mothers, allied with a tendency for the ICSI babies to be delivered slightly earlier.

The researchers, led by a team at the Royal Free Hospital in London, wrote: "Although our study showed that young children conceived after ICSI in the UK were healthy, by their nature children of this age have limited skill repertoires, hence the need for continuing assessment.

"The evidence to date suggests that the majority of children conceived after ICSI are healthy and developmentally normal."

However, not everyone is yet convinced.

Professor Chris Barratt, from the University of Birmingham, told BBC News Online that a study of 208 babies was "simply not enough" to provide reassurance.

He said: "We need bigger groups of children, perhaps 10,000 or so, and we need to follow them for much longer periods after birth.

"We have to understand that ICSI is still an experimental procedure."

Another UK-wide trial, involving every fertility clinic conducting ICSI, is being set up to provide a larger trial.