A jury has been told it must be sure a man fatally injured a 12-week-old boy during a "crucial 15-minute period" to find him guilty of murder.
Kane Mitchell, 31, denies killing Teddie Mitchell in St Neots, Cambridgeshire in 2019.
Defence barrister Matthew Jewell QC said prosecutors allege that he killed Teddie while the boy's mother was out.
He said jurors must be sure any injuries were caused during the "brief" period she was away from their house.
The jury at Cambridge Crown Court has been told Mr Mitchell was not Teddie's father, but was living with the boy's mother Lucci Smith, and in evidence she said they raised him as if he were Mr Mitchell's son.
Prosecutors allege Teddie was "subjected to a violent assault" on 1 November 2019 and died 10 days later.
'Screaming in pain'
In his closing speech, Mr Jewell said Ms Smith was out for about 15 minutes on the morning of the alleged attack.
"If you are to convict Kane Mitchell... you would have to be sure that it was during that brief period of time that he caused the injuries to Teddie," he told the jury.
The court has heard the injuries "would have led to Teddie screaming in pain... up to a point when he would have become unconscious".
Mr Jewell said there had been no independent witness of anything seen or heard that morning that assisted in determining when the injuries were inflicted.
Ms Smith, 29, is also on trial and she denies causing or allowing the death of a child, and of cruelty to a child, by not calling for an ambulance sooner.
Both defendants are jointly charged with causing or allowing serious harm to a child between 16 August 2019, the day Teddie was born, and 31 October.
The trial continues.