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A woman in Belgium is the first in the world to give birth to a baby using transplanted ovarian tissue frozen when she was still a child, doctors say.

The 27-year-old had an ovary removed at age 13, just before she began invasive treatment for sickle cell anaemia.

Her remaining ovary failed following the treatment, meaning she would have been unlikely to conceive without the transplant.

Experts hope that this procedure could eventually help other young patients.

The woman gave birth to a healthy boy in November 2014, and details of the case were published on Wednesday in the journal Human Reproduction.

Bone marrow transplant

The woman, who has asked to remain anonymous, was diagnosed with sickle cell anaemia at the age of five.

She emigrated from the Republic of Congo to Belgium where doctors decided her disease was so severe that she needed a bone marrow transplant using her brother's matching tissue.

But before they could begin the bone marrow transplant, they needed to give her chemotherapy to disable her immune system and stop it from rejecting the foreign tissue.

Chemotherapy can destroy the ovarian function, so they removed her right ovary and froze tissue fragments. At that time, she was showing signs of puberty, but had not yet started her periods. Her remaining ovary failed at 15.

Ten years later, she decided she wanted to have a baby, so doctors grafted four of her thawed ovarian fragments onto her remaining ovary and 11 fragments onto other sites in her body.

Baby boy

The patient started menstruating spontaneously five months later, and became pregnant naturally at the age of 27.

The gynaecologist who led to restore the patient's fertility, Dr Isabelle Demeestere, told the ����ý the patient was very stressed during the procedure because it was her only option to have a child, but that now she "is of course very happy and is enjoying her new life".

Dr Demeestere said it was now hoped the procedure could help other young people, especially given there is an increasing number of long-term survivors of haematological diseases diagnosed in childhood.

She said it was suitable for those who were at high risk of ovarian failure, such as survivors of treatment for lymphoma, leukaemia and sarcoma.

She said thousands of people had now undergone the procedure to freeze tissue and in Dr Demeestere's clinic, 20% of them were children.

"However, the success of this procedure requires further investigation in very young pre-pubertal girls, as our patient had already started puberty even though she had not started menstruating," she explained.

The risks involved

She also warned that it would only be suitable for patients at high risk of ovarian failure, because the procedure itself carries risks such as damaging the removed healthy ovary or reintroducing malignant cells at the time of transplant.

Professor Adam Balen, chairman of the British Fertility Society, welcomed the news.

"One would anticipate that young ovaries should have lots of eggs in them, the concern was whether those eggs might develop to maturity, if the ovarian tissue was taken at such a young age and frozen and then re-implanted," he told the ����ý.

"So, this is proof of that concept... it's very important information."

About 40 babies have already been born across the world using frozen ovarian tissue taken from older women.