French nanny Sophie Lionnet could have drowned in the bath either following a fall or a punch in the face, a court has heard.
The 21-year-old's charred remains were found on a bonfire in the back garden of her employers' home in Wimbledon, south west London, last September.
Sabrina Kouider, 35, and Ouissem Medouni, 40, have admitted perverting the course of justice by trying to dispose of the body but deny her murder.
The fashion designer and her partner have both claimed to have been sleeping at home when she died.
However, Medouni's initial account told of an "accident" in the bath during an "interrogation".
The prosecution allege the couple tortured Miss Lionnet into a confession over false claims she colluded with Kouider's ex-boyfriend, former Boyzone band member Mark Walton.
Giving evidence, a Home Office pathologist was unable to confirm the cause of the au pair's death because of the severe burns to the body.
But Dr Charlotte Randall refused to rule out a blow to the head, strangulation or drowning, the Old Bailey heard.
She told jurors how Miss Lionnet had fractured five ribs and her breast bone up to three days before her death.
More recent injuries included bruises, a "painful" broken jaw and possible cheek bone fracture.
On the difficulties of examining Miss Lionnet's remains, she said: "The body was severely, extensively burned due to fire damage.
"I had recent experience of mass fatality burned bodies in London and it was equivalent to that.
"The burn damage affected the whole body. There was only a small amount of skin left. The muscles were cooked and charred. A large part of the skull was missing, the brain was exposed."
Prosecutor Richard Horwell QC asked: "How impaired was your post-mortem because of the fire damage?"
She replied: "It was very impaired. The hair, the scalp and the cranium were missing."
However, Dr Randall found blood in Miss Lionnet's nostrils indicating blunt force trauma, possibly from a "blow or punch".
Cross-examining for Medouni, Orlando Pownall QC said: "You cannot say what caused the bruises or the fractures beyond the assertion it was some form of blunt impact?"
Dr Randall agreed they could have been from a punch, a blow with a blunt instrument or from a "fall or a series of falls" in the bathroom.
Mr Pownall went on to suggest the jaw fracture could have been caused by "a fall to the side of the bath, against a tap or some other blunt area within the bathroom".
He said Miss Lionnet could have fallen unconscious after suffering a broken cheek and jaw.
He went on: "You cannot say whether the cause of death was drowning. There were certainly appearances that might be consistent with that being the case?"
Dr Randall replied: "Yes."
On the possibility of strangulation, she agreed there was no sign of damage to the voice box.
The pathologist also agreed Miss Lionnet could have suffered bruises to her arm from a "grip"and further bruises to her back from impact with a tap in the bath.
She confirmed it was impossible to say whether an injury was caused by a man or a woman.
Icah Peart QC, for Kouider, reminded the court of Medouni's initial defence statement in which he admitted punching Miss Lionnet in the bath.
He said: "Mr Medouni says that he forced Sophie Lionnet into a bath and started to interrogate her.
"He says he forced her head under water and held it there repeatedly. Medouni states he punched Miss Lionnet in the face, her head then went backwards and hit the tiles and as a result, she slipped under the water and fell unconscious.
"He dragged her out of the bath and tried to resuscitate her but was unsuccessful."
Jurors have heard Medouni has since rejected this statement, saying he only made it to protect Kouider.
His later statement says he found Miss Lionnet in the bath after his partner woke him to say she had stopped breathing.
The trial continues.