Family anger at boy's sentence for killing pregnant mum

A Perth teenager who hurled a concrete block at a car, fatally injuring a pregnant mother has been sentenced to five years' detention but with time already served could be released in less than two years.

Noongar woman Diane Miller's family screamed out in grief and anger in court over the sentence handed down on Friday.

The 17-year-old boy, who cannot be named for legal reasons, pleaded guilty to unlawfully killing Ms Miller, 30, in December last year.

He admitted lying in wait behind a wall after a car park melee involving weapons on November 29 and throwing a 1.9 kilogram brick at a car.

It flew through the passenger side window and hit Ms Miller, who was five moths' pregnant, in the head causing catastrophic injuries.

She died in hospital, along with her unborn child, amid an outpouring of grief in the Noongar community.

The teen, who was initially charged with murder before agreeing to plead guilty to the lesser charge of manslaughter, fled the scene before handing himself in to police several hours later.

Ms Miller's family members wept and verbalised their anger over her death in Perth Children's Court as Judge Hylton Quail read the facts of the "tragic" incident.

"You were impulsive ... You didn't stop to think if it was a bad idea," he told the boy.

"You persisted with the plan to use the rock."

Judge Quail said the boy's traumatic life and brain damage had contributed to his decision to throw the lump of concrete.

He also accepted that the teen was genuinely remorseful and co-operated with police.

However, he said the Ms Miller was an innocent and blameless victim of a cowardly act.

"Most significantly she had a baby and was 20 weeks' pregnant," Judge Quail said.

"You took her life and deprived baby Lloyd of his mother and you deprived Ms Miller's unborn child of life.

"You will have to live with the consequences for the rest of your life."

The boy was sentenced to five years' detention at Banksia Hill Juvenile Detention Centre and must serve a minimum of 26 months before applying for supervised release.

After the sentence was handed down Ms Miller's family's anger boiled over as they were ushered from the courtroom by security officers.

Outside the building, Ms Miller's brother Malcolm Clifton said he was outraged by the sentence and the teenager should have also been punished for the death of his sister's unborn child.

"There's justice for you, what a load of bulls*** ... I'm sick to the guts with the law system," he said.

"He knew what he was doing when he picked up the brick. Everyone knows what they are doing when they pick up a weapon. They intend to use the f***ing thing."