LOS ANGELES (AFP) - A six-day old premature baby has become the youngest infant to receive a heart transplant at a US hospital, doctors and his proud parents said 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. The parents didn't expect him to survive.

"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.

"But he came out fighting," she added in a statement released by the hospital.

The baby is recovering in hospital after being born on Jan 5.

The mother and father, Chris Crawford, were first alerted to the problem after a prenatal exam at 20 weeks showed a defect in the tiny baby's heart, called dilated cardiomyopathy.

It was confirmed four weeks later. "The left ventricle was huge for a 24-week-old," paediatric cardiologist Dr Christopher Lindblade told the Arizona Republic, adding: "It was massive."

Doctors planned for a birth at 36 weeks, but the mother went into labour at 33 weeks.

Within hours of the birth, the baby was screened to see if he was healthy enough for a heart transplant. Baby Oliver was put on a national transplant waiting list on Jan 9, and two days later a viable heart became available.

The operation lasted for 10 hours, after which the infant had an incision from his belly to his chest.

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

The baby remains in hospital and his lungs remain weak but is doing "amazingly well" at 2.7kg, according to the hospital.

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