FOUR forensic pathologists have told the Folbigg baby deaths inquiry that smothering a child can leave signs on a baby's body but can also occur and "leave no trace".
The four include Victorian Institute of Forensic Medicine Professor Stephen Cordner, whose report that there was nothing to suggest the four Folbigg babies had been killed, let alone smothered, was in a petition to the NSW Governor in 2015 seeking a judicial review of the Folbigg case.
Kathleen Folbigg was convicted in 2003 of killing her four babies at Singleton between 1989 and 1999 and is serving a minimum 25 year jail sentence.
An inquiry called by NSW Attorney General Mark Speakman was told on Tuesday that blood and froth on the mouth of Caleb Folbigg, the first of the Folbigg children to die at only 19 days old, was not unusual in Sudden Infant Death Syndrome (SIDS) cases.
"The presence of frothy bloody fluid is commonplace," said forensic pathologist Professor John Hilton, who performed an autopsy on Laura Folbigg, the fourth baby to die, and gave evidence at her mother's trial.
Professor Cordner, Professor Hilton, Professor Johan Duflou and Newcastle Deprtment of Forensic Medicine Dr Allan Cala all answered yes when asked by counsel assisting the inquiry, Gail Furness, SC, if "smothering can leave signs but it also may not leave signs".
Professor Hilton told the inquiry that research by University of Newcastle Professor Cecelia Blackwell linking minor infections, an inflammatory response in babies and SIDS was "a very interesting theory".
Dr Cala told the inquiry it was yet to achieve "broad forensic medical agreement".
Three of the four Folbigg babies had mild infections in the period before their deaths.
Dr Cala, who gave evidence at the 2003 trial, told the inquiry he would always have concerns if three babies in one family died sudden, unexplained deaths.
"I'd be extremely cautious," he said.
"If I was on the receiving end of a third death, assuming the other two had been investigated elsewhere... I wouldn't call it a category of SIDS (Sudden Infant Death Syndrome) because there were three deaths," Dr Cala said.
"I would have to ask the question is something being missed here, or has a comprehensive investigation of any of them been done and has something been missed?"
Professor Cordner, that there is nothing from a forensic pathology viewpoint to suggest any of the four Folbigg babies, Caleb, Patrick, Sarah and Laura, had been killed, let alone smothered by their mother.
Professor Cordner found there were identifiable natural causes of death for Patrick and Laura, and natural causes are a plausible explanation for the deaths of Caleb and Sarah.
In his report Professor Cordner said much of the forensic pathology discussed at the trial was "misconceived, based as it is on a flawed understanding of asphyxia".
"Asphyxia is not a helpful word in forensic pathology, is not understood in a uniform way, is not a diagnosis and is not diagnosable," he wrote.
"Yet the word is at the core of the main question asked repeatedly by the prosecution during the trial : 'Did this child/these children suffer an acute catastrophic asphyxiating event?'"
Professor Cordner said the question was not capable of an answer if it was intended to be a technical question in forensic pathology.
In his report Professor Cordner said the trial included evidence given under circumstances where a "default diagnosis of murder" was discussed.
"The fact that an infant can be smothered without leaving signs, the misunderstanding of asphyxia (in particular that it is a diagnosis and/or that it can be diagnosed), and there being no families in the literature with three or four SIDS, contributed significantly to a homicide hypothesis which in fact has little forensic pathology content."
The three other forensic pathologists told the inquiry they agreed with Professor Cordner about his views on the use of the word asphyxia as "not a diagnosis".
Questioned by counsel assisting the inquiry, Gail Furness, SC, about whether SIDS always meant a natural death, Professor Cordner said it did not.
"We all understand whenever we use the word or term SIDS, there's always the possibility there may be an unnatural explanation, or there may be a natural explanation we can't uncover," Professor Cordner said.
On Tuesday Kathleen Folbigg continued to follow the inquiry via an audio-visual link from jail after choosing not to attend the hearing at Lidcombe Coroners Court in person. She will give evidence from April 17.
The inquiry continues.