A 5-month-old baby killed by his mother suffered severe brain injures, bruising to his head and neck, and had meth in his system, a Perth court has heard.
Melissa Louise Bulloch, 27, has pleaded guilty to the manslaughter of her son Lachlan, who was found dead at his Hamilton Hill home in January 2014.
Prosecutor Carmel Barbagallo told the WA Supreme Court during a sentencing hearing on Thursday that Bulloch's abuse of the baby in the months leading up to his death included blowing cannabis smoke into his face, leaving him in a car, and shaking and swearing at him.
The court heard the Department for Child Protection was also involved with the family and police listened to Bulloch's phone calls and conversations after the baby's death.
On one occasion, she said: "I'm sorry Lachlan ... for killing you, my sweetheart. That freaked me out."
In a letter read to the court, Bulloch said she was beyond sorry for what happened and wished she could take it back.
"I live every day knowing that I won't be a mother to him and watch him grow up," she said.
Defence counsel Ian Hope outlined Bulloch's mental health issues and said she did not remember what happened because of amnesia.
But Ms Barbagallo labelled Bulloch a manipulator who deliberately chose not to disclose details.
She submitted that the crime was at the upper end of the scale of seriousness for manslaughter and if there was any remorse, it came very late.
Ms Barbagallo referred to the maternal grandmother's victim impact statement, which described her "unimaginable heartache" and the crime as something she will never forget.
Justice John Chaney had earlier in the hearing denied a request from the grandmother to read her statement in court.
After hearing the submissions, Justice Chaney said it was a difficult sentencing exercise and he needed more time to reflect on all the material.
Bulloch will be sentenced on October 3.