A forensic pathologist says it would be "highly unusual" for a baby’s rib fractures to be caused by being rolled on by his mother.
A prominent sportsman, who has interim name suppression, is accused of causing the injuries, but has pleaded not guilty to a charge of injuring with reckless disregard (and an alternative count of assault).
The Crown case is the man squeezed or crushed the baby during a moment of frustration when the child’s mother was at the gym.
Days later, the child was found to have 13 broken ribs and a fractured collarbone.
The defence has suggested multiple other possibilities, including vitamin D deficiency causing weak bones, injury during birth or the baby’s mother rolling on to him in her sleep.
Yesterday, forensic pathologist Dr Leslie Anderson said she had seen many infant deaths caused by unsafe sleeping arrangements, but they usually did not have serious injuries.
"Accidental rib fractures are very rare due to the pliability of infants’ bones and the force required to fracture them," she said.
"Non-accidental injury is thought to be the most common explanation for rib fractures in infants."
An image of the baby co-sleeping with his mother was shown to the witness.
She thought it was an unsafe sleeping arrangement, but did not believe the fractures would be caused if the mother had rolled on to the baby because of the soft surface they were on.
"I still wouldn’t expect the child to have rib fractures in this scenario. Is it possible? Maybe. It would be highly unusual.
"If I saw this photo and then found a bunch of rib fractures I would be raising major alarm bells."
Earlier in the day, paediatric endocrinologist Prof Ben Wheeler told the court an ongoing lack of vitamin D could lead, in the most extreme circumstances, to rickets.
Rickets led to fractures in about 10% of cases, but that was usually in toddlers, who suffered breaks to longer bones in the arms or legs, Prof Wheeler said.
"Rib fractures are unusual. Multiple rib fractures are extremely, extremely unusual," he said.
In any case, he was confident having reviewed X-rays, biochemical testing and other evidence the baby did not suffer from rickets.
Prof Wheeler said there was no research to say multiple rib fractures could be caused by simple vitamin D deficiency — something that was a common issue among babies born in the South.
He also pointed out it was "highly unlikely" the injuries were caused at birth.
"If [the child] had severe bone fragility you’d expect to see ongoing fractures after first admission," Prof Wheeler said.
Prof Wheeler said the child’s presentation was consistent with non-accidental injury caused by "substantial force".
The trial, before Judge David Robinson, continues today.
