Women in their late thirties undergoing IVF treatment could be limited to a single embryo without affecting their chances of conception, according to research that also promises to cut the rate of twin and triplet births.

Fertility experts in Finland found that transferring a single embryo was as likely to result in a birth in women aged 36 to 39 as younger recipients of fertility treatment. The quality of the embryo rather than the age of the woman was the most important factor.

The findings, published in the journal Human Reproduction, may encourage wider acceptance of single-egg transfers and reduce the risks associated with multiple births. The treatment will also limit the chances that women who want only one child will have twins or triplets.

In Britain, the common practice is to transfer up to three embryos per cycle in women aged 39 and over, according to the Human Fertilisation and Embryology Authority. Below that age, a maximum of two embryos can be transferred.

The study focused on 1,224 cycles using fresh embryos and 828 frozen embryo transfers in women aged 36 to 39. The researchers found that, in the 36-39 age group, 33 per cent of the women achieved preg- nancy after a single-embryo transfer — similar to the rate in younger women.

The birthrate was also similar at 26 per cent, compared with the 27 per cent to 30 per cent in younger women.

The researchers also analysed the results for women who underwent more than one cycle of treatment using fresh and frozen embryos, known as the cumulative pregnancy rate.

Hannu Martikainen, of the University of Oulu, and the senior author, said: “What we demonstrated for the first time was that the pregnancy rate, and in particular the cumulative pregnancy rate, was very similar in the age group 36 to 39 to that obtained previously in younger women.

“This suggests that embryo quality is the most important parameter in the outcome and that selection for elective single-embryo transfer should be based on embryo quality rather than the age of the woman.”

The study also found that women had a higher birth rate if they had several cycles with one embryo per cycle rather than cycles with two embryos.

When the cumulative rate was analysed, the single- embryo transfer results were actually better than with double-embryo transfers, with a pregnancy rate of 54 per cent and birth rate of 41.5 per cent in the women having a single- embryo transfer, compared with a pregnancy rate of 35 per cent and birth rate of 26.7 per cent for double-embryo transfers. There was a multiple birth rate among the single-embryo transfer group of less than 2 per cent, compared with nearly 17 per cent in the double embryo group.

Dr Martikainen said that it showed that a single-embryo transfer could be as successful in women up to 40 as it was in younger women and that it also cut the risk of multiple births compared with double-embryo transfers.

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