A six-day-old premature baby has become the youngest infant to receive a heart transplant at a US hospital, doctors and the infant's proud parents said on Thursday.

Baby Oliver Crawford underwent the operation at Phoenix Children's Hospital in Arizona after being born seven weeks ahead of schedule with a heart defect that meant his parents didn't expect him to survive.

"The doctors had very little hope that he would survive the pregnancy, and when my waters broke at 33 weeks, we were prepared to deliver a still born baby," said Caylyn Otto, the infant's mother.