Six-day-old premature baby successfully given a heart transplant after a 10-hour surgery. Getty Images

A six-day-old premature baby has become the youngest infant to receive a heart transplant in the US, doctors revealed on Thursday (12 February).

Baby Oliver Crawford was born seven weeks earlier than expected with a heart defect at the Phoenix Children's Hospital in Arizona.

"The doctors had very little hope that he would survive the pregnancy, and when our water broke at 33 weeks, we were prepared to deliver a still born baby," said Caylyn, the infant's mother, in a statement released by the hospital.

"But he came out fighting."

Crawford's parents were first informed of his condition at the prenatal exam at 20 weeks that revealed a defect in the little baby's heart, called dilated cardiomyopathy.

The condition was confirmed four weeks later.

"The left ventricle was huge for a 24-week-old. It was massive," Dr. Christopher Lindblade, a pediatric cardiologist, told the Arizona Republic.

Shortly after birth at 33 weeks, doctors examined Crawford's health to see if he could undergo the heart transplant.

After being put on a national transplant waiting list four days after his birth on 9 January, a viable heart was available two days later.

The entire surgery took 10 hours and an incision was later made from his stomach to his chest.

"From our understanding Oliver is the youngest recipient in the nation. He received his transplant at 34 weeks and 3 days gestation," said his mother, reported France24 News.

Baby Crawford is still recovering at the hospital after being born on 5 January.

His lungs remain weak, but according to the hospital he is doing "amazing" at 6.1 pounds.

"It is amazing, after expecting the worst, Oliver is indeed a miracle," said Crawford's father.