Using a powerful microscope to select the healthiest-looking sperm improves fertility treatment success rates, say Italian scientists.
They boosted the pregnancy rate using ICSI - where a single sperm is injected into the egg - from 26.5% to 39.2%.
And, writing in Reproductive Biomedicine Online, they said among men who had failed using conventional ICSI the success rate doubled.
But a fertility expert said UK clinics would need more evidence of benefits.
"The main obstacle to implementing it would be the cost, and the number of staff needed to do it "
Many men with fertility problems do not produce normal, healthy sperm, which can "swim" towards the egg.
This means that ICSI, which stands for intracytoplasmic sperm injection, is the only way to produce an embryo.
Evidence suggests that sperm which are misshapen, or poor swimmers, are less likely to produce a viable pregnancy, and sperm selected for conventional ICSI are inspected under a microscope for signs of this.
The new technique, originally pioneered by Israeli scientists, is a refinement of that selection process, using a microscope with five times more power than those usually pointed at a sperm sample.
This allows sperm size and shape to be assessed in more detail.
The researchers, led by Monica Antinori, from the Raprui Clinic in Rome, looked at 446 couples in which the male was sub-fertile, but the woman did not have any obvious problems.
Expensive
The new method, called IMSI, or intracytoplasmic morphologically-selected sperm injection, consistently outperformed conventional ICSI.
In men who had already had at least two failed attempts using ICSI, the success rate was 29.8% per attempt compared with just 12.9% among those persevering with the older technique.
The downside, however, is that IMSI, as offered by the Italian clinic, is about twice the cost of ICSI, which already can cost more than £3,000 per cycle in the UK.
No UK clinics offer the technique as yet, but Dr Allan Pacey, the secretary of the British Fertility Society, and a senior lecturer in andrology at the University of Sheffield, said that this might change in the next few years.
"At the moment we've got one really good study here which suggests that it works, but we do need more.
"The main obstacle to implementing it would be the cost, and the number of staff needed to do it - if you were to put it into every clinic tomorrow, there would not be enough trained scientific staff to carry it out."
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