While IVF has improved greatly since the birth of Louise Brown 30 years ago, fertility doctors still have few tools for choosing which embryos they should implant into the womb.

More than half of all embryos, and as many as 75 per cent of those produced by women over 40, suffer from chromosomal abnormalities that prevent normal development. A good way of identifying them would allow the selection of normal embryos for transfer, and so improve success rates without implanting more than one.

A pre-implantation genetic screening (PGS) test is licensed at eight British clinics for patients with a history of failed IVF cycles or miscarriage. However, it is unreliable and has been severely criticised by some scientists. It involves creating IVF embryos and growing them in the laboratory for two to three days, until they reach eight cells. One is then removed for genetic testing and only chromosomally normal embryos are replaced in the womb.

The current approach to genetic analysis, however, can screen at most ten chromosomes, and since humans have 23 pairs, some abnormalities are missed. The biopsy procedure is hard to conduct on fragile eight-cell embryos, and some may be damaged. Last year a randomised controlled trial of PGS, led by Sebastian Mastenbroek of the University of Amsterdam, found that it actually reduced birth rates, probably because of these problems. Since then, many practitioners have become sceptical about PGS.

In the new approach the test is conducted on blastocysts, embryos grown for five days until they have 100 to 150 cells. These are larger and stronger, allowing cells to be removed more safely. A second advance is an analysis technique called comparative genomic hybridisation (CGH), which is simpler and can examine all the chromosomes. As CGH takes several days to provide results, blastocysts must be frozen. New technology makes this practical.

The results from the Oxford and Colorado team suggest that blastocyst CGH is effective, though a larger study with a control group will be needed to change doctors’ minds. The theory is sound, however, and the approach looks promising.

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