By Caroline Ryan
BBC News Online health staff in Berlin
A woman has become pregnant after having an ovary tissue transplant for the first time, it has emerged.
Ovarian tissue was transplanted into the woman
The breakthrough gives hope to thousands of cancer patients whose treatment can make them infertile.
It may also potentially help women who want to give themselves another chance at motherhood after the menopause.
Doctors from Universite Catholique de Louvain in Brussels are treating the woman. News of her pregnancy was revealed at the European Fertility Conference in Berlin.
The baby, a girl, who was conceived naturally, is due at the beginning of October.
Doctors across the world have been working to achieve a successful pregnancy in ovarian transplant patients.
Other teams have been able to create embryos, and have attempted to help women become pregnant through using IVF.
But this is the first time a successful pregnancy has occurred in a woman who has had an ovary transplant.
The patient, who is 32, was diagnosed with advanced Hodgkin's lymphoma in 1997.
Prior to chemotherapy, some of her ovarian tissue was removed and frozen. One ovary was left inside her body.
When she was declared cancer-free in April 2003, the ovarian tissue was transplanted back into her body, just below her existing ovary.
Four months later, she was found to be menstruating and ovulating normally. She is now around 25 weeks pregnant.
However, it is not yet clear if the egg which was fertilised came from the grafted tissue, or from her ovary which could have begun to work again independently.
Professor Jacques Donnez, who led the research, told RTBF Radio 1 in Brussels: "She is pregnant. She lives a life which she could never hope she would have been able to live.
"It's her child genetically, growing from her tissue, and she fell pregnant completely naturally."
Professor Kutluk Oktay, of Cornell University, New York, who has pioneered work in this field told BBC News Online: "We need to see more details about this work, but it would be a landmark finding.
He added: "If you leave women alone, there is a chance of spontaneous recovery of ovarian function.
"But there is a viable possibility that the pregnancy came from this graft."
He said many patients found facing up to the possibility they may never have children was almost as hard to come to terms with as the news of their diagnosis - so the possibilities offered by this treatment were "tremendous"
"It helps them to cope with cancer and to have a much more positive attitude to the circumstances that they are in. It takes some of the burden away."
Dr Oktay said the approach could also help other women with blood, kidney and joint diseases who were also treated with cancer drugs.
He said there was still too little evidence to recommend healthy woman should freeze ovarian tissue which they could have transplanted back into their bodies after their menopause to give them another chance of having a baby.
But he added: "I wouldn't recommend it now. But if you found out that there was a 30% pregnancy rate - as with IVF - why not?"
Dr Yding Andersen, from the University Hospital of Copenhagen, who is waiting
to see if an ovarian transplant patient he has treated becomes pregnant, said:
"It's definitely a breakthrough, absolutely a breakthrough, and it has huge
implications for a lot of women around the world."
Simon Davies, spokesman for the Teenage Cancer Trust, said: "It's great news, it's fantastic.
"This is a new option and it's going to be worthwhile for cancer patients, particular young ones, and will give them more choices in the future."
But Josephine Quintavalle of the Centre for Reproduction Ethics, said: "This technique should not be used lightly.
"I sincerely hope it is not used as a lifestyle choice for deciding when you want to have children."
Professor Jack Scarisbrick, of the charity Life, added: "It is likely this technique will be misused.
"It is an important development. But trying to defer the menopause is not the right way for it to be used."