Schoolgirl Lucy McHugh died from "catastrophic bleeding" after she was stabbed and slashed 27 times with a knife, a court has heard.
Stephen Nicholson, 25, is accused of repeatedly stabbing the 13-year-old girl in the neck and upper body at Southampton Sports Centre in July last year.
Pathologist Dr Basil Purdue told Winchester Crown Court that Lucy suffered 20 stab wounds and seven sliced wounds to her face, upper chest and forearms caused by a single-bladed knife.
He said: "The cause of her death was catastrophic bleeding from a series of 11 injuries to the right side of the neck that punctured her right carotid artery in three places.
"This would have been rapidly followed by further sharp injuries to the face, neck, chest and lower forearms."
Dr Purdue told the court that one sliced injury to Lucy's left wrist area appeared as if it could have been inflicted deliberately in a bid "to divert suspicions away from homicide".
The trial has previously heard that Lucy had sent a message to Nicholson claiming she was pregnant following their year-long "secret sexual relationship".
However Dr Purdue said on Monday that blood tests showed that she was not pregnant.
Giving evidence, the partner of Lucy's mother said Nicholson had been an occasional lodger at their home in Southampton.
Richard Elmes told the court he had not thought that Lucy was sexually active and he and her mother, Stacey White, confronted the teenager about comments made at school about her being pregnant.
Lucy said to the pair she told her friends that her mother was pregnant, Mr Elmes added.
He told the court that he had considered Lucy and Nicholson as being like part of a "normal family" with "petty" arguments.
He said that he believed Lucy had a "schoolgirl crush" on Nicholson and in May 2018, he walked in on "Lucy giving him a hug" before the defendant pushed her away.
Mr Elmes said that a week before her death, Nicholson and Lucy had an argument and had another row which was like a "war zone" on 23 July, during which she shouted at the defendant: "I have got a hold on you anyway."
"I told him that basically he had to grab his stuff up and he had to leave because the situation wasn't getting any better," Mr Elmes said.
He described Lucy as a "lovely, bubbly, bright, intelligent girl" who "knew what she wanted and how to get there".
Mr Elmes said he had known Nicholson since he was six years old and the defendant kept six snakes and a tortoise at their house.
Nicholson, a carer and tattoo artist, of no fixed address, denies Lucy's murder and three counts of rape against her when she was 12.
He has also pleaded not guilty to two charges of sexual activity with a child against Lucy on multiple occasions when she was 13, and a charge of sexual activity with a child in relation to another 14-year-old girl in June 2012.
The trial continues.