It was impossible to say what degree of force was needed to cause the head injury that killed 10-month-old baby Chloe Murphy, the Supreme Court has heard.
In her autopsy report, forensic pathologist Dr Melissa Baker said Chloe's injuries could have been caused by shaking or some type of impact or a combination of both.
Chloe was unconscious and struggling for breath when her parents, Tony and Phurithee, picked her up from babysitter Ketapat Jenkins' Kensington townhouse just after 11pm on December 3, 2010, after going out for dinner and a movie.
Chloe was rushed to the Royal Children's Hospital and found to have suffered a fractured skull and three fractures to her left arm. She died two days later.
Ms Jenkins, 30, a mother of two, has pleaded not guilty to one count of child homicide, which is effectively manslaughter.
When later questioned by police, Ms Jenkins denied having shaken or dropped Chloe while looking after the baby and suggested Chloe's parents may have been to blame.
Dr Baker's autopsy report, which was read to the jury by her colleague, forensic neuropathologist Dr Linda Iles because Dr Baker is seriously ill, revealed the cause of Chloe's death was a head injury.
"It should be noted that in the absence of accurate biomechanical models and a clear mechanism of injury, assessment of the degree of force required to produce this constellation of injuries is not possible," Dr Baker said in her report.
"The exact mechanism of the head injury cannot be determined with certainty but may be due to shaking or impact or a combination of both."
Dr Baker said determining when the head injury occurred was obviously pivotal in this case and it was the neurosurgeon's opinion that the injury occurred within 12 hours of Chloe being operated on.
But Dr Baker said the medical literature on "infants sustaining severe head trauma suggests that the onset of symptoms occurs at the time or shortly after the traumatic event".
Questioned by defence barrister Peter Morrissey, SC, Dr Iles agreed with the proposition that the pattern of injuries suffered by Chloe could have been sustained in a complex fall where there was more than one impact.
On Monday, Royal Children's Hospital ophthalmologist Dr James Elder gave evidence that he believed Chloe's injuries were most likely to have been caused when her head had been repeatedly moved backwards and forwards.
The trial, before Justice Lex Lasry, continues.