Doctors in Belgium tried to restore the fertility of a woman using ovarian tissue removed from her when she was just 13.

It worked: The 27-year-old woman delivered a healthy baby boy late last year. The Belgian doctors who reported the case Tuesday in the journal Human Reproduction write that it is the first live delivery of a baby conceived after a transplant of childhood ovarian tissue.

"This is an important breakthrough in the field because children are the patients who are most likely to benefit from the procedure in the future," Isabelle Demeestere, a gynecologist and research associate at Erasmus Hospital in Brussels, said in a news release. "When they are diagnosed with diseases that require treatment that can destroy ovarian function, freezing ovarian tissue is the only available option for preserving their fertility."

In this case, the woman (whose identity the authors did not reveal) had severe sickle-cell anemia as a child and needed to undergo chemotherapy for a bone marrow transplant — a process that would effectively leave her unable to conceive. So at 13 years and 11 months, before she began menstruating, doctors removed her right ovary and some ovarian tissue. Her left ovary eventually failed, and she needed hormone therapy to menstruate.

A decade later, she wanted to have a baby. So doctors stopped the hormone therapy, thawed some of her frozen ovarian tissue and then grafted fragments onto her failed ovary and elsewhere in her body. Within five months, the tissue grew follicles with maturing eggs and she began having regular periods on her own. But her partner, the doctors write, had infertility issues.

Then, at 27, the woman conceived naturally.

"After more than two years post-transplantation, the patient had a spontaneous pregnancy with a new partner and spontaneously delivered a healthy boy in November 2014," the authors write.

The woman's ovaries still function normally and she could try to have another baby, the doctors said. "She also has the possibility of undergoing a second transplantation with the remaining frozen tissue if the graft stops working, as we didn't transplant all the ovarian tissue the first time," Demeestere said.

Another fertility medical first took place just months before the woman in Belgium gave birth. A 36-year-old woman born without a uterus gave birth to a baby boy in Sweden after receiving a womb transplant.

Although the newer case may signal hope to girls facing the prospect of procedures that would leave them unable to conceive, Demeestere cautioned that further research is needed to determine whether fertility could be restored using ovarian tissue removed from prepubescent girls. The woman in the Belgium case, the authors write, showed signs of starting puberty a few years before doctors harvested her ovarian tissue.

Also, the lead doctor said that this highly invasive procedure isn't appropriate as a method to start menstruation in girls or adults and that such patients should instead consider hormone therapy.

"We think, at present, that cryopreserved ovarian tissue should be used only for fertility restoration in patients at high risk of ovarian failure," Demeestere said.