Khaylan Butler's life support was turned off on December 3, 2023, two days after being shaken and dropped into a cot by Maddison Butler's then-partner Daniel John Ridgers.
Ridgers in November pleaded guilty to manslaughter after initially being charged with murder.
The 26-year-old shook Khaylan five times after he was "sooky" following a visit to his grandfather's house.
Maddison Butler told the court how she watched the life leave the "perfect little body" of her son. (Ethan James/AAP PHOTOS)
In an initial interview Ridgers told police he had dropped Khaylan after losing his balance while stepping on a toy car.
He later admitted being "tired and drained" when he shook Khaylan "really hard" after an attempt to get him settled.
"I don't know why I shook him. If I could take it back I would in a heartbeat," he said during the interview.
"I just flipped. I was just having a bad day."
Ms Butler came home from work about 1am to find her son struggling to breathe and clammy.
When she picked him up, his eyes rolled back, the Supreme Court of Tasmania in Hobart was told on Wednesday.
An MRI scan revealed Khaylan had suffered an unsurvivable catastrophic brain injury, with an autopsy showing injuries consistent with "vigorous" shaking.
He also suffered bruising to his head from hitting the side of the cot after being dropped, as well as a tear in his mouth.
"When they turned off Khaylan's life support I watched the life leave his perfect little body," Ms Butler told the court.
"The physical and mental pain of leaving that room is indescribable … I never knew you could hurt like that.
"Khaylan should be 22 months now. Speaking his first words. Moving from crawling to walking.
"But what should be, is now a living nightmare filled with horror."
Ms Butler said Khaylan was perfect and sweet with a cheeky grin that everyone fell in love with.
"Sometimes I feel I failed him … how could I not have known the danger he was in?" she said.
"Time does not heal. It just takes me further away from the last time I held my baby."
Ridgers, not Khaylan's biological father, has been in custody since the incident and will return to court on December 20.
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