Babies begin producing antibodies to different blood types between 5 months and 2 1/2 years of age - it varies widely from child to child, says West, now at the University of Alberta's Stollery Children's Hospital. Only a few of the 90 or so mismatched heart transplants performed worldwide have occurred past a child's first birthday, the oldest in a 30-month-old in Britain.

Still, age is just a rough marker for antibody production, West stresses. Blood tests to check antibodies are the real key.

Hence the new US policy, adopted last fall and to go into effect later this year. It expands use of mismatched hearts up to age 2, as long as antibody tests show the toddlers are candidates.

For now, many transplant centers are like Webber's, trying their first mismatched transplants in babies before working up to toddlers.

Last March, Connor Geddes of Erie, Pa., became Pittsburgh's first of five such transplants. His heart's left side was too small to pump. Doctors said Connor wouldn't live long enough to await a heart that matched his Type A blood, but they had a heart from a Type B donor available.

"It still amazes me," says Carrie Geddes. "When we talk to people, friends, and tell them, nobody really realizes that can happen."

Eleven months later, Connor shows no sign of rejection and happily totters after his older brothers. His tracheotomy tube - from lungs were weakened by heart-pumping machines while he awaited the transplant - is to be removed soon, and the scar on his chest is barely visible.

UNOS' Sweet calls the heart policy "a first step in what we really think is a long process in improving wait-list mortality for all children."

At a first-of-its-kind meeting March, UNOS will take a hard look at hurdles to improving child organ donation, especially for babies and toddlers. One problem is that when grieving parents consent to a donation, organs aren't always recovered, perhaps because the local transplant center didn't immediately see a good recipient, Sweet says.

"It doesn't mean there is no patient suitable for that organ in the whole United States," he says. "There are organs out there that if we find the right recipient, they could be transplanted. Even if it's one at a time, I'm willing to work on that."

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