How do you make a baby from a frozen ovary? Like this.

This week a 27-year-old woman made history when doctors revealed that she had given birth to a healthy baby boy last November. What’s so special about about that? Well, the woman had first undergone an ovarian transplant using tissue extracted from her own body and frozen a decade earlier. Even crazier, the tissue was taken at age 13, before she had started menstruating.

Hailed as a medical milestone and detailed in the journal Human Reproduction, the Belgian patient’s procedure offers new hope for kids facing chemo and other harsh radiation therapies—hope that they might be able to preserve their future fertility against the damaging effects of the therapies and have kids of their own someday.

Richard Anderson, a professor of clinical reproductive science at the University of Edinburgh who collaborated on some of the testing of the woman’s ovarian tissue, spoke with Fusion and explained the mechanics behind the pioneering procedure. But first, the backstory.

Arts and grafts

The patient, who has asked to remain anonymous, was diagnosed with sickle cell anemia at the age of 5. By the time she was 11, a bone marrow transplant was necessary. Two years later, doctors were able to carry out a Hematopoietic stem cell transplantation thanks to a sibling who was a match.

Bone marrow transplants are incredibly strenuous—the girl underwent chemo and had to take immunosuppressive drugs for 18 months to be sure her body did not reject the blood transplants.  Because child cancer survivors—particularly those who have undergone bone marrow transplants—face a huge risk of premature ovarian insufficiency (or loss of the function of ovaries before age 40), her parents opted to remove her right ovary and cryopreserve it.

Ovarian grafting has been successfully performed before, but the tissue has only been taken from adults. While the patient had begun puberty when the tissue was extracted, she had not yet experienced menarche (the first period), so it was uncertain if her tissue could someday successfully integrate with her tissue as an adult.

How does it work?

So how did a 13-year-old’s pre-menarche ovary create a baby 13 years later?

First, the patient’s right ovary was removed in a procedure known as a “laparoscopic oophorectomy” in 2001, explained Anderson. The doctors then froze 62 tiny fragments of the ovary, each containing two to six follicles. (Ovarian follicles are the basic functional unit of the ovaries. Each houses an immature egg and releases the egg during ovulation. Follicles also secrete menstrual hormones.)

About ten years later, when the patient decided she was ready to have a baby, the doctors thawed a few of the ovary fragments and transplanted them back into her. They grafted four fragments onto the left ovary, 6 onto the perineal bursa (an abdominal lining by the stomach) and 5 subcutaneously.

“After the pieces of tissue are put back, blood starts to grow into it, which happens naturally when you put a piece of tissue back.  That just happens by itself,” Anderson told Fusion.

“And then the follicles within the ovaries start to grow,” he said. “If they grow, they start producing hormones, and then that starts normal ovulatory cycles which is what happened in this lady. Just normal hormonal changes like in any other woman.”

Despite the fact that the ovary tissue was taken before the patient had fully gone through puberty (or gotten her first period), the tissue integrated successfully. Within five months of the transplant, the woman had her first natural period, and she experienced regular monthly menstruation after that. More than two years after the transplant, she had successfully conceived. And shortly thereafter, she had successfully given birth to a healthy baby boy.

Because the patient had already begun puberty, said Anderson, the integration was a bit easier. The next step in ovary transplants? Taking cryopreserved tissue from a child who has not yet even begun puberty and seeing if it successfully grafts.