It is possible a single stamp from a child could have led to the heart injury that resulted in toddler Liam Fee's death, a court has heard.

Pathologist Dr Paul French was giving evidence for a second day at the trial of Rachel Trelfa or Fee, 31, and her civil partner Nyomi Fee, 28, who deny murdering Trelfa's son Liam.

Jurors have heard how Dr French carried out a post-mortem examination on the two-year-old's body two days after his death at a house in Fife in March 2014.

He told the High Court in Livingston how the toddler had a ruptured heart and he recorded the cause of death as blunt force injury to the torso.

Dr French, 38, said "severe force" would have been needed to cause the injury.

During cross-examination from Mark Stewart QC, defending Fee, he was asked that if the child had been stamped on in a "sudden, sharp manner" to the abdomen, would it have generated the sort of "compressive force" that led to the heart injury.

Dr French replied: "That is one possibility, yes."

Mr Stewart asked: "That event, if it occurred, was capable of being executed by a single person.

"Yes, that's possible," Dr French said.

"That single person might have been a four stone child?" asked Mr Stewart.

"That's possible," the witness said.

During re-examination of his evidence, advocate depute Alex Prentice QC, prosecuting, put it to Dr French that it was also "equally possible an adult could have done it".

The witness agreed.

"It's not just a push or a punch in the stomach, is it?" asked Mr Prentice.

"No, it's severe force," said Dr French.

The murder charge alleges that the couple assaulted Liam at a house in Fife on various occasions between March 15 and March 22, 2014.

They also face a catalogue of allegations of wilfully ill-treating and neglecting two other young boys, who cannot be named for legal reasons, over a period of more than two years.

The pair are accused of falsely telling others, including police, that one of the other young boys was responsible for Liam's death.

They deny all the charges.

On day four of the trial, jurors also heard from Professor Anthony Freemont who specialises in diseases of the bones.

He was sent samples from Dr French for analysis after he found fractures to Liam's upper arm and thigh.

Prof Freemont, 63, said the evidence showed "two fracturing events" and in his opinion these were three to five days previously and less than six hours before his death.

He said he did not find any evidence of an underlying bone disease which might have made Liam more prone to fracture or a lack of vitamin D.

The court then heard from Dr Ralph Bouhaidar, a consultant forensic pathologist who carried out the post-mortem examination alongside Dr French and had attended the scene at the house in Fife the day after Liam's death.

Dr Bouhaidar, 41, was asked by the advocate depute if the rupture of Liam's heart would have caused death "almost instantaneously", to which he agreed.

The trial, before judge Lord Burns, continue on Friday.