ICSI (intracytoplasmic sperm injection) involves using a microscopic needle to inject a single sperm into an egg recovered during an in vitro fertilisation procedure.

The technique is used when a known male infertility problem prevents normal fertilisation from occurring.

It has led to the birth of many health babies.

Dr Gerald Schatten and colleagues at the Oregon Regional Primate Research Centre examined rhesus monkeys conceived using the technique.

They used imaging to watch the fertilisation process, and saw irregularities in the way the egg was fertilised.

Writing in the journal Nature Medicine, they warn that the technique might itself damage the chromosomes which carry the genes - the building blocks of life.

Chromosomal damage can cause birth defects, including mental retardation.

Birth defects

Out of 14 attempts, five rhesus monkeys were born.

Two healthy female babies were born without complications; one male baby was stillborn, and two other females had evidence of eye defects.

Dr Schatten's team was, in fact, trying to show that their technique might work for preserving endangered species.

They also noted that some infertile men had genetic irregularities that could contribute to their infertility - the idea being that nature protects the gene pool from these defects by preventing conception in the first place.

In a 1996 report, David Page and colleagues at the Massachusetts Institute of Technology said tests on 35 infertile men showed two were missing a gene, or possibly group of genes, known as the azoospermia factor (AZF).

If they had babies conceived by ICSI, the babies would inherit the defect.

And last summer a report in the Lancet medical journal said ICSI babies were a little more likely to show mild delays in mental development.

About 40% of infertility cases are due to male defects. Sometimes sperm cannot penetrate the egg on their own.

In addition, 3 or 4% of men produce just a few viable sperm - too few for normal fertility but enough to make them candidates for ICSI.

ICSI 'poses no higher risk'

Writing a commentary in the same journal, in vitro fertilisation pioneer Dr Robert Edwards, of Human Reproduction Journals in Cambridge, Britain, said the rate of birth defects in babies born by ICSI was "similar to that for natural conception".

He said: "I don't think overall there's a vast risk in them."

Dr Edwards said abnormal embryos die very quickly and do not develop into babies.

"Very soon after ICSI began we knew there would be anomalies, but they just die out - they don't go on," he said.

"ICSI is so simple and so successful that all well-equipped IVF centres should continue to develop it."

Edwards said he weathered much criticism in the early days of laboratory fertility work.

"We had astonishing scare stories from very senior scientists," he said.

"All those have cleared. We had enough data in mice and rabbits before we even approached humans."

Edwards said the benefits of fertility treatment far outweigh any risks.