BOSTON, Massachusetts (WCVB/CNN) - They are twins, but they don't share a birthday.

In fact Alexandre is more than three weeks older than his brother Ronaldo, but both boys are thriving after a rare and risky delayed delivery that has exceeded every expectation.

"This was unique in many ways, including that both twins are doing well," Dr. Sabrina Craigo said.

The challenge began when mom Linda DaSilva went into labor only 24 weeks into her pregnancy. Doctors at Tufts tried to stop the contractions but 4 days later, a very premature Alexandre was born.

"We were praying to hear his cry, and we did. And it was a relief because knew there was 50 percent chance of not surviving," mom said.

Doctors placed baby No. 1 in intensive care and told mom and dad it was time to consider keeping baby No. 2 in mom's womb a little longer.

"I thought it was absurd! I said, 'I don't think … I didn't know this was possible.'"

But doctors explained a "delayed twin delivery" could be done, and Linda was a perfect candidate.

"He's like leave him there, he is fine; his heart is beating well, so OK. Then we start praying," dad Ronaldo Antunes said.

A healthy Ronanldo would be born 24 days later weighing in a full pound more than his big brother.