By Kate Kelland

LONDON (Reuters) - A Congolese-Belgian woman has become the first in the world to give birth to a healthy child after doctors restored her fertility by transplanting ovarian tissue that was removed and frozen when she was a child.

The woman, who was diagnosed with sickle-cell anemia when she was five and emigrated to Belgium at age 11, needed a bone marrow transplant.

Thinking of her future potential to have a family, the Belgian doctors decided before starting the treatment to remove the patient's right ovary when she was 13 years and 11 months old and froze tissue fragments.

Reporting the success June 9 in the journal Human Reproduction, Belgian doctors said it pointed to a future where children with serious illnesses such as cancer may find a way to have babies many years later.

"This is an important breakthrough in the field because children are the patients who are most likely to benefit ... in the future," said Isabelle Demeestere, a gynecologist and research associate at Belgium's Erasmus Hospital.

"When they are diagnosed with diseases that require treatment that can destroy ovarian function, freezing ovarian tissue is the only ... option for preserving their fertility."

While there have been reports of successful pregnancies after ovarian transplantation using tissue removed from adult patients, there have been none yet using tissue taken from girls before puberty.

This patient, who has asked to remain anonymous, had not started her periods when her ovary tissue was removed and frozen, although her doctors said there were signs she had started puberty with breast development at around age 10.

After undergoing chemotherapy, a bone marrow transplant and more than a year of treatment with immuno-suppressive drugs after developing graft-versus-host disease, her remaining ovary failed at the age of 15.

But 12 years later, after doctors successfully transplanted the thawed ovarian tissue the patient became pregnant at age 27 and delivered a healthy boy in November 2014.

However, Demeestere, as well as independent experts, cautioned that the procedure's potential success needs to be further explored for young, pre-pubertal girls.

"There had previously been uncertainty as to whether ovarian tissue taken from young girls would later on be competent to produce mature, fertile eggs, so today's case is both reassuring and exciting," said Adam Balen, a professor at the Leeds Centre for Reproductive Medicine.

He added: "We have to remember that many children who require chemotherapy are very ill and the surgery to remove ovarian tissue is no small undertaking."