Screening tests to identify embryos with the best chance of developing into healthy babies have the potential to transform IVF success rates for the better.

More than half of all embryos have an abnormal number of chromosomes but, while tests that aim to detect these abnormalities have been available for six years, their chequered performance has left many specialists sceptical of the whole idea.

The tests, known as pre-implantation genetic screening (PGS), involve removing a single cell from an embryo for analysis, so only normal embryos are implanted into the womb. In 2007 a randomised clinical trial at the University of Amsterdam found that PGS actually seemed to reduce the chances of pregnancy. Advocates of PGS are convinced the theory behind it is sound — although basic PGS looks at only 10 of the 24 chromosome types.

Scientists such as Dagan Wells, at Reprogenetics, and Simon Fishel, at the CARE fertility group, have developed more sophisticated tools, using a new technique called comparative genomic hybridisation (CGH), that can examine all 24 chromosomes. Pilot studies of CGH have been encouraging but the controversy is only going to be settled one way: randomised controlled trials. The acid test for embryo screening is about to begin.

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