Teen lay in wait for pregnant Perth mum hit by brick

Family members of a pregnant Perth mother fatally injured when a teenager hurled a concrete block through a car window have sobbed in court as they watched videos of the tragic incident.

The CCTV footage recorded at a suburban shopping centre was shown at the sentence hearing for a 17-year-old boy who has admitted unlawfully killing Diane Miller.

The 30-year-old Noongar woman died in hospital, along with her unborn child, several days after a brick hit her in the head as she sat in the passenger seat on November 29 last year.

The Perth Children's Court on Thursday heard the incident followed a heated verbal altercation in the shopping centre car park involving Ms Miller's partner, the teen and a group of his friends.

Prosecutor Clare Cullen said the teenager, who cannot be named for legal reasons, had "lay in wait" before hurling the 1.9-kilogram brick at the car as it drove away from the scene of the stoush.

She also said he had known Ms Miller, her infant son and three young women were in the car and he threw the concrete block anyway, increasing his culpability for the crime. 

"The concrete went through the window striking Ms Miller to the head," she said.

"It was cowardly."

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.

His lawyer, Kate Turtley-Chappel, said the boy "was emotionally aroused and he was fearing his and other's safety" when he threw the brick.

"(The defendant) threw the rock without thinking and ultimately it ended in the loss of human life," she said.

"He is quite frank in saying he wasn't thinking in terms of the outcome."

Ms Turtley-Chappel said the teen was extremely remorseful and had accepted he would be sentenced to a term of imprisonment.

"This is a young person who says 'I want to be here (and) I did wrong and I need to do my time'," she said.

"(He) showed immediate remorse when he heard this lady had got hurt. He fell to the ground and he cried. 

"He called police, he participated in an hour-long interview ... He never applied for bail."

She said the teenager had a traumatic upbringing and had been diagnosed with fetal alcohol spectrum disorder and attention deficit hyperactivity disorder.

"What we have is a young person removed from his mother at a young age and exposed to violence at a young age," she said.

"Now he sits in this courtroom accepting his fate and knowing he caused so much devastation."

The hearing continues on Friday when the teenager is expected to be sentenced.