A teenager accused of throwing a concrete block at a Perth mother, killing her and her unborn child, has had his charge upgraded to murder.

Diane Miller suffered a heart attack last week when the object was thrown through the open window of her car at a Karawara shopping centre.

The 30-year-old Noongar woman died in hospital after spending several days in a coma.

She was five months’ pregnant and also had an eight-month-old son.

The 17-year-old boy who allegedly threw the concrete block was initially charged with grievous bodily harm and breach of bail.

Western Australian police said on Thursday the charge had been upgraded to murder. The teenager is due to return to Perth Children’s Court on Friday.

Family members gathered last week at the shopping centre, where Ms Miller’s partner Phillip Edmonds described his shock at her passing.

“We all loved her as one and that’s what brought us together,” he said.

“That’s my baby girl and that was my baby inside.”

Police allege the boy was one of several teenagers involved in an altercation at the shopping centre.

“This is, quite simply, senseless violence by a bunch of teenagers,” Inspector Brett Baddock said.

“From what we can gather, it’s unprovoked. I think they’ve gone out looking for trouble and they’ve found it.”

Passersby tried to resuscitate Ms Miller before an ambulance arrived.

The family has remembered her as a happy and funny person, and urged the community to let the police investigation unfold and to refrain from any retribution.

-AAP