Jacques Donnez, a professor of gynaecology at the Catholic University of Louvain in Brussels, said it was too early to draw firm conclusions but the procedure gave hope to women who had not had an opportunity to freeze either eggs or ovarian tissue before undergoing damaging cancer therapy.

"This method is an option for women who have not had their ovarian tissue cryopreserved (frozen)," Donnez said on Thursday.

The procedure could be used between two unrelated women, as long as they had compatible tissue types and had swapped bone marrow, he said.

Doctors went on to produce two embryos from eggs recovered from Alvaro's ovaries using in vitro fertilisation (IVF), although neither developed into a viable pregnancy.

Donnez, reporting her case in the journal Human Reproduction, said it was not clear why the embryos ceased to develop but noted this also happened during normal IVF treatment.

Alvaro herself is confident she will become pregnant.

"I feel this is what I needed to do and I will have children one day," she told Reuters in a telephone interview.

She was inspired to seek an ovarian tissue transplant after reading in 2005 about a 24-year-old American woman who gave birth to a healthy baby after receiving a similar tissue donation from her twin sister.

Since then, other women have also become pregnant after receiving ovarian tissue from their twins -- but Alvaro is the first example of a successful transplant between non-identical sisters.

Their different genetic make-up added a layer of complexity to the procedure. However, tests showed their genetically different cells could coexist successfully and Alvaro's sister had also previously donated bone marrow to her, so there was no problem with rejection.

A simpler option might have been for Alvaro's sister Sandra, who is three years younger than her, to have donated an egg. This idea, however, was rejected by both women.

"I wanted something of my own and I felt if I went for egg donation it would be someone doing it for me," Alvaro said.

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