Embryos can be screened for chromosomal abnormalities before implantation.SPL

Dutch researchers have questioned the effectiveness of a technique that many fertility specialists believe improves the chances of pregnancy. Preimplantation genetic screening (PGS) harms the chances of motherhood for women older than 35, they claim.

PGS involves removing a single cell from a three-day-old embryo and checking that a selection of chromosomes is normal. The technique is widely regarded as giving the best chance of success for older women undergoing fertility treatment, whose embryos are at greater risk of chromosomal abnormalities.

But a study of more than 400 women aged 35 to 41 found that women receiving PGS had significantly lower pregnancy and live birth rates. Twelve weeks after the introduction of embryos, just 25% of those given PGS were pregnant, compared with 37% of controls. The team, led by are reported in the New England Journal of Medicine¹.

Removing a cell may damage embryos more than was previously realized, the study's leader, Sebastiaan Mastenbroek, told the European Society for Human Reproduction and Embryology's annual meeting in Lyon, France. Or, he suggests, many three-day embryos may be 'mosaics' of cells with different chromosomes, meaning that a clear screen does not necessarily guarantee a healthy embryo.

"In theory it's a high-tech technique that sounds very plausible, but still you should be careful, considering the data that are accumulating," says Mastenbroek, who works at the Academic Medical Center in Amsterdam.

He recommends that doctors skip PGS and choose embryos on the basis of how well-formed they look. At three days old, a healthy embryo should look like a neat ball of eight cells.

Balancing act

Not everyone agrees. "Although morphology is useful, it's a very blunt tool," says Dagan Wells, who studies early embryos at Yale University in New Haven, Connecticut. "With most embryos that look healthy, it's impossible to tell which have chromosomal abnormalities. I still think there's a benefit to PGS in terms of reducing miscarriage."

Wells adds that the chemical methods used by the Dutch team to screen the embryos are not the best available.

Removing cells from an embryo harms its chances of implantation, but avoiding chromosomal abnormalities can be worth the risk, Wells argues: "PGS has to more than compensate for the damage. In the hands of a good lab that's possible. It's a fine balancing act." One problem is that standards for embryo biopsies vary from country to country and even lab to lab, he adds.

Roughly 4,000 courses of PGS are declared in Europe and the United States each year, although this is likely to be a massive underestimate of the true number. The technology is a money-spinner — one British clinic charges £1,950 (US$3,900) for the procedure.

PGS is also offered to women who have experienced in vitro fertilization (IVF) failure or persistent miscarriage. Researchers at the Free University of Brussels in Belgium are carrying out a similar study of women younger than 35. It is not known how PGS affects their chances of successful pregnancy.

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• References

  1. Mastenbroek, S. et al. New Engl. J. Med. 357 , 9-17 (2007).