A single high-force impact to a 14-week-old baby boy's head caused all his extensive fatal injuries, a medical expert witness for the defence told a Tauranga court.
Dr Terence Donald, a paediatric forensic physician, made the comment in the High Court at Tauranga on Friday at the trial of a Bay of Plenty man accused of murdering the infant.
Richard Royal Uddin was pronounced dead on June 7, 2016, after his mother rushed her son to Tauranga Hospital for treatment.
Baby Richard sustained severe injuries, which included multiple skull fractures.
The man accused of his murder, who has interim name suppression, has accepted responsibility for causing the injuries, but denied he meant to kill the baby.
Crown prosecutor Anna Pollett earlier told the jury the level of force used against the child showed the defendant had murderous intent or meant to injure the baby knowing it was likely to cause death.
Dr Donald said he disagreed with the prosecution's two medical experts as to how many impact sites there were on the child's skull.
He remained satisfied there was only one impact site after reviewing forensic pathologist Dr Dianne Vertes' autopsy report, the CT scans and 3D imaging of the baby's skull.
Dr Donald said one episode of severe force to the head would explain all the injuries.
"This is not something minor. This was massive force enough to cause all the injuries we are talking about."
Markings or bruising on the cheek and right ear were not impact points but consistent with an adult's left hand grabbing the face of the infant "forcefully", he said.
"To me that's not caused by a slap or a punch but a grab ... and you can see the shape of a left-hand thumb print."
His testimony follows that of Dr Vertes, who revealed her post-mortem findings to the jury on Thursday and said there were at least three impact sites and two to three separate depression fracture injuries.
The cause of death was due to "blunt force craniocerebral trauma" injuries to both sides of the skull and to the brain after the boy's head impacted with a "hard surface".
Dr Vertes said there were two depression fractures with distinct impact points - one each on the right and left side of his head - and "possibly" a third smaller depressed fracture.
The baby also had a misshapen flattened posterior head, and a 13cm by 9.5cm weave-patterned injury across the back of his scalp, she said.
Dr Vertes said the left depression fracture was consistent with the boy's head hitting the carpet area of a floor.
"High-force impact caused the depression fractures ... It was either something hitting the head of the child or the infant's head hitting something."
There were injuries to the brain, red contusions on the boy's forehead, bruising in his right ear, and four bruises on the deceased's left cheek, she said.
When asked by Ms Pollett what caused the cheek bruising, Dr Vertes said it could have been a "punch, a slap, or squeezing".
"But the squeeze would have to be pretty strong to cause other deep underlying injuries."
Dropping the infant on to a floor or "accidentally" hitting a wall would not have caused these types of injuries, she said.
"This is a violent force. This is significant force which is the better word."
Paediatric radiologist Dr Russell Metcalfe said he agreed with Dr Vertes because there were multiple fractures revealed in the CT scans and 3D imaging of the boy's skull.
Dr Metcalfe said as well as depression fractures, "gross" brain swelling, and significant bleeding, there were lots of smaller fractures - "too many to count".
It was difficult to say with 100 per cent certainty, but he "doubted very much" that one impact alone would have caused such severe injuries.
Dr Metcalfe had seen lots of blunt force head injury trauma cases at Starship Children's Hospital over 23 years and ranked this infant's injuries in the "severe" non-accidental category.
The trial continues on Monday, with the prosecution and defence closing addresses, before the jury retires to consider its verdict.