A teenager admitted yesterday that he bashed his three-week-old baby to death during the few minutes he was alone with the premature infant at a Bunbury hospital last year.
The 16-year-old, who cannot be named, hit his baby's head against a hard surface at least twice at Bunbury Regional Hospital last February.
After examining the baby, neuropathologist Vicki Fabian described the boy's head and brain injuries as the "most severe" she had seen in an infant.
The baby's father was charged with murder but prosecutors yesterday accepted his guilty plea to manslaughter in Perth Children's Court.
The infant was born six weeks premature but was a healthy little boy and was just two days away from being discharged from hospital.
The father hit both sides of the infant's head with "considerable force" when they were left alone in a room for between three and 10 minutes.
His partner returned to find the teen holding their unresponsive and pale baby.
Outside court, the baby's mother said she hoped justice would be done for her son.
"There is not a day that goes by that I don't think of him," she said.
The baby was "limp like a rag doll" and had milk coming from his mouth and nose when he was taken from his father's arms.
He died nine days after being transferred to Princess Margaret Hospital with skull fractures and bleeding on the brain.
Prosecutor Bruno Fiannaca said the baby's father was the only person who could explain exactly what had happened to his child.
He said there were "at least two separate and deliberate physical actions".
The court was told it was possible the child's head was stomped on while he was lying on his side on the ground.
"In order to fracture an infant's skull, considerable force is required," Mr Fiannaca said.
The teen told police he bumped the baby's head on a wall or door with enough force to hear a "pop" while trying to leave the room.
But Mr Fiannaca dismissed this explanation as implausible and inconsistent with the forensic evidence.
The baby's father sat with his head in his hands as the facts of the shocking case were read out in court.
He was remanded in custody for sentencing on March 23.