You can call them Manitoba's "wonder twins."

Zoey and Zane Espayos turned two Aug. 30, a milestone and medical marvel because the pair has overcome a genetic blood disorder that should have claimed their lives in the womb.

"We are really grateful and happy," their mother Reina Espayos told CBC News from Hamiota, Man., through tears. "We didn't expect it that they [would] reach this age."

The Espayos family celebrates another milestone with their two-year-old twins. "They're now starting to walk and start talking ... they're eating by themselves."

When Espayos was 27 weeks pregnant, the twins went into heart failure. Doctors diagnosed them with alpha thalessemia, a disorder in which the blood does not produce haemoglobin necessary to carry oxygen throughout the body.

"The normal thing that would happen is that the babies would pass in utero and they would be born a still birth," said Dr. Geoff Cuvelier, a hematologist and pediatric oncologist with CancerCare Manitoba.

He was the physician who diagnosed the twins in heart failure. Immediately he asked doctors if it was possible to perform blood transfusions on the baby girls who were still inside their mother.

"And the answer to that was they were going to give it a try," he said. "It's very technically difficult. There are two umbilical cords and they're very, very small. And the perinatologist needs to ... put the needle into the little umbilical vein and start transfusing these babies blood in utero."

If the transfusions were successful, once they were born they would need blood transfusions every week.

"Ultimately over the long-term we had to develop some other strategy so we took them to bone-marrow transplant."

First twins in the world to have transplant

In the fall of 2014, the twins elder sister Zachi was chosen as a bone marrow match. Doctors say Zoey and Zane are the first twins in the world with this disorder to have the transplant.

Zoey went first but her body rejected the bone marrow. That was only a minor setback, though, because within a few months an anonymous donor was found and Zoey went for a second transplant.

It was successful, so her sister Zane was given the same marrow during a transplant in April 2015.

"We staggered [the procedures] because the two kids look identical and they both have first names that begin with 'z' and we didn't want to mix the two kids up," Cuvelier said. "I know that sounds very funny, but that's actually the major reason."

Cuvelier said they wanted no room for medical error, and they did not want the family to end up with two sick girls at the same time if the transplant failed.

To everyone's hope the procedure was a success, the marrow is allowing the blood to produce hemoglobin and the girls are, in Cuvelier's words, "cured."

"It's absolutely wonderful," he said. "They're wonderful little kids and they're developing just perfectly and it's very gratifying as a physician."

Cuvelier said the prognosis is excellent and expects the girls to go on to lead normal lives.

"It's special ... we're really now starting to move forward," said Espayos.