An operation designed to preserve women's fertility before they have cancer treatment might also extend their lifespan, doctors have speculated.

Cancer therapy can leave women infertile, but some patients have had ovary transplants afterwards to restore their fertility. The ovary tissue is either collected and frozen before their treatment begins, or is donated by another woman.

In a series of animal experiments, researchers found that ovary transplants rejuventated mice and extended their lifespan by more than 40%. In the study, ovaries from young mice were implanted into older mice. Researchers are now asking whether the equivalent operation extends the life of women.

Fewer than a dozen babies have been born around the world after their mothers received an ovary transplant. The first was born in London in 2008, to a 39-year-old woman who conceived naturally after receiving an ovary from her twin sister.

Doctors at a meeting of the European Society of Human Reproduction and Embryology in Rome, where the research was announced, said ovarian transplants might rejuvenate and extend the lifespan of mice by changing levels of hormones in their bodies.

"At present, ovarian transplants are performed with the aim of preserving a woman's fertility after cancer treatment for instance, or of extending her reproductive lifespan," said Dr Noriko Kagawa at the Kato Ladies Clinic in Tokyo, who was one of the researchers. "However, the completely unexpected extra benefit of fertility-preserving procedures in our mouse studies indicates that there is a possibility that carrying out similar procedures in women could lengthen their lifespans in general."

Dr Kagawa stressed that far more research is needed to investigate whether ovarian transplants extend the lives of women, particularly as none of the women has yet reached old age.

Arne Akbar, an expert on the immune system and ageing at University College London, said: "What people in the field of ageing research will want to know is, if this is a real effect, what is the biological mechanism?"

In one set of experiments, Kagawa's team removed both ovaries from mice when they were around 140 days old, and implanted them in six older mice aged around 525 days. Before the operation, the older mice were too old to be fertile.

In further experiments, single ovaries from young mice, around 170 days old, were implanted into eight older mice, aged around 540 days old.

All of the mice who received transplants became fertile again, but more striking was the effect the operation had on their longevity. Mice that received two ovaries lived for an average of 915 days, while those that had one ovary implanted lived for an average of 877 days. Their average lifespan was expected to be 548 days.

After their transplant operations, the aged mice began to behave as though they were much younger. "They showed interest in male mice, mated and some had pups. Normally, old mice stay in the corner of the cage and don't move much," Dr Kagawa said.