Scott Austic is serving life in prison for the murder of 34-year-old Stacey Thorne, who was stabbed 21 times at her Boddington home, southeast of Perth, in 2007.

She was 22 weeks pregnant with Austic's baby and it was alleged at his trial, which was based largely on circumstantial evidence, that he stabbed her because he did not want the child.

Austic previously failed in an appeal and is facing a new hearing agreed to by the attorney-general amid concerns police may have planted evidence, including a cigarette packet at the home and a 10cm folding knife found in a paddock.

UK forensic pathologist Richard Shepherd, who has 35 years experience, told the WA Court of Appeal on Tuesday some wounds measured between 13cm and 15cm, which meant they were "very unlikely" to be caused by the weapon.

"I don't think that knife caused those wounds," he said.

Dr Shepherd, who has conducted about 23,000 post-mortem examinations, told reporters outside court he could not comment on whether he believed there had been a miscarriage of justice.

"From the pathology point of view there are some discrepancies but that doesn't necessarily mean everything falls," he said.

"Some of the wounds described by Dr (Karin) Margolius are so much longer than the length of the blade of the knife that I have great concerns that the knife caused those wounds."

Austic's mother Robyn told reporters Dr Shepherd was "fabulous to listen to" in court.

She also said she spoke to her son after the first day of the hearing on Monday and "he thought it was a good day".

Australian Associated Press