A leading paediatrics expert has discredited some of the evidence that was used to convict Kathleen Folbigg for killing her four children in NSW.
A second judicial inquiry is looking at Ms Folbigg's convictions for killing Laura, Sarah, Caleb and Patrick between 1989 — 1999.
She maintains all four children — under the age of two — died of natural causes on separate occasions in her Hunter Valley home.
The 55-year-old has five years left to serve of a 25-year sentence after being found guilty of three counts of murder and one of manslaughter.
Some experts say new scientific evidence shows a rare gene mutation that may have caused the deaths of Laura and Sarah.
The mutation was not found in Caleb and Patrick.
Professor Peter Fleming, a world-renowned paediatric intensivist, told the inquiry he had personally investigated 200 child deaths and was part of a team investigating another 600 child deaths.
Retired Chief Justice Tom Bathurst KC asked the witness if the children could possibly be smothered with no signs of injury, as was the Crown case at her 2003 trial.
"It is possible but very hard to accept and I'd be very surprised," Professor Fleming said.
Mr Bathurst noted that at a 2019 judicial inquiry, experts claimed "the absence of any signs of smothering supported the hypothesis of smothering".
Professor Fleming responded that he had much more experience in this field of research than the witnesses at the trial and subsequent inquiry.
"A paediatric intensivist was not asked their opinion on this in either the trial or the first inquiry," he said.
"I may be an outlier, but I have greater experience than the others who gave evidence," he said.
He explained children who are being suffocated "wriggle" and "fight vigorously" and have signs of injury inside their mouth caused by their teeth.
Ms Folbigg's children did not have these injuries, he said.
"I would find it very hard to believe that somebody could suffocate them by putting something over their face or obstructing their airways and leaving no marks.
"I find it very hard to envisage any way you could do that without putting pressure on the lips and teeth.
"I would find that extraordinary."
Professor Fleming said he was aware of other expert reports that suggested myocarditis was linked to the death of the sons Caleb and Patrick.
Paediatric neurologist and federal MP Monique Ryan told the inquiry Patrick most likely died of natural causes.
Dr Ryan, the former head of a neurology department at Melbourne's Royal Children's Hospital, said Patrick probably had an underlying medical condition that caused seizures and his death.
"It is a clinical presentation I've seen many times in my career," she said.
But she wouldn't rule out that he died from a brain injury caused by lack of oxygen.
The inquiry continues.
