But that also raises the odds of multiple births, which can be dangerous for the mother and babies.

Scientists at the University of Oulu in Finland have shown that a single, good quality embryo is as likely to result in a pregnancy in women aged 36-39 as transferring two embryos and it is safer because it results in fewer multiple births.

"It is more a question of quality than quantity," Dr Hannu Martikainen, who headed the research team, said in an interview.

"This is the first time this has been shown in this age group of women," he added.

Martikainen said his results suggest that the quality of the embryo may be more important in determining the outcome of IVF than the woman's age.

"In the age group 36-39, we nowadays use single embryo transfer in about 40 percent of cases. As a result of this, a multiple pregnancy rate of less than 10 percent has been achieved," he said.

Success rate

The researchers analysed the success rate of more than 1,224 cycles of fertility treatments in women aged 36-39 using fresh embryos and 828 in which the embryo had been frozen. A third of the women became pregnant after a single quality embryo transfer, which is similar to the rate in younger women.

The success rate among the older women was 26 percent, which was only slightly less than the results seen in an earlier study of women under 36.

The researchers, who reported the results in the journal Human Reproduction, said success rates using one quality, fresh embryo was similar to women who had two embryos placed in the womb.

But the multiple birth rate was less than 2 percent in the single embryo group compared with nearly 17 percent the other group.

"This tells us that as well as elective single embryo transfer having the potential to be as successful in women up to 40 as it is in younger women, it also reduces the risk of multiple births compared with double embryo transfer," Martikainen added.