The team then drew bone marrow from Ms. Obar’s hip bones, and along with Elianna’s second transfusion — about a week after the first — they dripped in about 50 million of her mother’s stem cells, an enormous number compared with usual transplants.

Stem cells home in on the bone marrow, and the reason for such a big dose was to give the new cells the best possible chance to displace Elianna’s defective ones and produce a healthy blood supply for her. She had three more transfusions over the next few months, until her birth.

In one sense, the experiment has already been a success: there have been no adverse effects to mother or baby, so the treatment seems safe.

“I’m thrilled that it was safe and it was feasible,” Dr. MacKenzie said, adding it was also important “to get the message out that fetal transfusions for alpha thalassemia are lifesaving.” She expects to perform transplants on a few more patients, see how they fare and then decide how to proceed.

So far for Elianna, in her first three months, there has been no obvious benefit from the transplant. Like all children with her blood disorder, she needs a transfusion every three weeks.

But tests have found some of her mother’s stem cells in her blood. Whether they will start to help her is unknown. If they do not, her parents could eventually opt for a bone-marrow transplant to cure the disease and free her from a lifetime of transfusions.

In that case, if some of her mother’s cells persist, they may make it possible for her to accept another transplant from Ms. Obar with less chemotherapy than the procedure usually requires.

Those decisions are a long way off.

“Elianna’s doing great,” Ms. Obar said. “I’m not disappointed at all. If it works, great. If it didn’t, we’re O.K. with it. We’ll celebrate all the little accomplishments. I’m glad we did it.”