A study in the journal Annals of Oncology published Wednesday describes how the baby boy was born to a 34-year-old French woman who had been treated with chemotherapy for breast cancer.

Before the treatment began, doctors removed seven immature eggs from her ovaries and used a technique called in vitro maturation (IVM) to allow the eggs to develop further in the laboratory.

Up to now, there have been no successful pregnancies in cancer patients with eggs that have undergone IVM and freezing.

Some children, however, have been born as a result of IVM immediately followed by fertilization and transfer to the patient.

Michael Grynberg, head of the Department of Reproductive Medicine and Fertility Preservation at Antoine Beclere University Hospital near Paris, recalled becoming aware of the 29-year-old patient's case.

"I offered her the option of egg freezing after IVM, and also freezing ovarian tissue," he said in a statement.

"She rejected the second option, which was considered too invasive a couple of days after cancer diagnosis."

So-called cryopreservation of ovarian tissue is an experimental method in which the outer layer of an ovary—which contains immature eggs—is taken out of the body and frozen for future use.

In the case of the French patient, ultrasound revealed that there were 17 small, fluid-filled sacs containing immature eggs in her ovaries.

But using hormones to stimulate the ovaries to ripen the eggs would have taken too long and might have made her cancer worse, leaving retrieval of the immature eggs and freezing as the best option.

After 5 years, the patient recovered from breast cancer, but she was unable to conceive naturally. The chemo had made her infertile.

Six of the eggs that had been frozen 5 years earlier survived the thawing process, and five were successfully fertilized.

One of these fertilized eggs was transferred to the patient's womb, and she gave birth to a healthy baby boy—named Jules—on July 6, 2019.

"This success represents a breakthrough in the field of fertility preservation," Grynberg said.