Single embryo transfer more effective with blastocysts

Culturing embryos to the blastocyst stage in the laboratory could increase the success rate of in vitro fertilisation (IVF) treatment involving the transfer of a single embryo, Belgian doctors report. A study published in the New England Journal of Medicine suggests that in women under 36, transferring a single five-day-old embryo results in a higher pregnancy rate than transferring a single three-day-old embryo. An accompanying commentary calls the findings 'encouraging', but says it is unclear whether they would apply to older women undergoing treatment.

Concerns over the increased risk of medical problems associated with multiple births following fertility treatment have lead to calls for routine single embryo transfer (SET). However, many patients feel that such a move would drastically cut their chances of becoming pregnant - particularly in countries that provide no or limited funding for IVF.

In Belgium, infertile couples are eligible for up to six free cycles of IVF treatment, provided that in women under 36, only one embryo is transferred in the first two cycles. The authors of the latest study, based at the Centre for Reproductive Medicine in Brussels, wanted to see if growing embryos for five days in the lab before transfer would allow them to assess embryo quality more accurately, and so increase the chances of success with SET.

The study involved 351 women aged 35 or younger who were undergoing their first or second embryo transfer during 2003-2004. Half the participants were randomly assigned to receive a single three-day old embryo, and the other half received a single five-day-old embryo. The pregnancy rate in the group who received blastocysts was 33 per cent, compared to 23 per cent in the group that received the less developed embryos. The authors suggest that the better outcome is probably due to the fact that fewer embryos with genetic abnormalities develop to the five day stage, compared with those that manage to grow to the three-day stage.

In an accompanying editorial, Laura Schieve - an expert on IVF based at the US Centers for Disease Control - said the findings suggest that 'single blastocyst transfer might be an optimal treatment'. However, she pointed out that the results couldn't be generalised to include everyone undergoing IVF - less than half of all US women having fertility treatment are under 36, for example. She also cautioned that the study focussed on pregnancy rates as a measure of success, and was not intended to assess any possible harm that might come to the embryo as a result of extended culturing.