However, a major analysis of research into the safety of intracytoplasmic sperm injection (ICSI)has found that the risk is small.

The technique, developed a decade ago to help men who either have very few sperm, or whose sperm cannot swim properly, ruling out conventional IVF treatment.

ICSI is a technique whereby a single sperm is injected directly into an egg in the laboratory.

For many couples who otherwise could not achieve a pregnancy the slight increase in risk associated with these techniques will be acceptable

Dr Ulla-Britt Wennerholm, Sahlgrenska University Hospital,Gothenburg

The new study, presented at the European Society of Human Reproduction and Embryology annual conference in Vienna, has raised the possibility that it might lead to genetic abnormalities because it transmits genetic faults from the father to the child.

There have been concerns that the technology increases the risk of a serious genetic problem because it bypasses the natural selection that comes into play following sexual intercourse - under normal circumstances a defective sperm would stand no chance of fertilising an egg.

Researchers from Sahlgrenska University Hospital in Gothenburg, Sweden, have carried out a comprehensive analysis of studies into the safety of ICSI. They also presented their results at ESHRE conference.

Dr Ulla-Britt Wennerholm said most studies found that ICSI was no more dangerous than other IVF techniques.

She said that studies that did suggest a significantly increased risk tended to be flawed.

Dr Wennerholm said: "What is most important for parents to know is that the absolute risk for a serious congenital malformation or chromosomal aberration associated with IVF or ICSI is small.

"For many couples who otherwise could not achieve a pregnancy the slight increase in risk associated with these techniques will be acceptable."

Mental development

A separate study presented at the conference found that the mental development of ICSI children is on a par with other IVF children and children born without recourse to fertility techniques.

A team from the University of Sydney and the Royal North Shore Hospital in St Leonards, New South Wales, carried out IQ tests on 97 ICSI children, 80 IVF children and 110 controls at the age of five.

The team had been concerned about possible long-term effects of ICSI after an earlier study found that one-year-old ICSI children appeared to be at increased risk for delayed mental development.

The researchers therefore reassessed these children, along with other ICSI and control children.

They found that by five years, there was no significant difference between the groups for full-scale IQ.

Lead investigator Professor Garth Leslie said: "Whilst we found ICSI was an independent predictor of delayed development at one year, this was no longer the case at five years when other demographic factors were more important.

"These results provide reassurance that the ICSI procedure does not produce children at increased risk for below normal long-term cognitive development."

However, a further, small-scale study hinted that some ICSI children did inherit chromosomal abnormalities from their fathers.

It was suggested that the consequences of these might be not only fertility-related defects, but there might also be a link to autistic spectrum disorders.

However, the tiny number of children involved in the study means the results cannot be widely applied, particularly as the cause of such disorders is still far from established.

Boys less likely

Another study, by the French research institute INSERM, found that couples who have a baby via ICSI seem to have less chance of having a boy. The reasons are not yet clear.

The likelihood of having a boy was lowest if the sperm was taken from directly inside the male reproductive tract.

It was higher if ejaculated sperm was used to fertilise the egg.

What is not clear is whether this is related to a higher death rate among male embryos, or to decreased fertility among the sperm that carry the genetic code for a boy.