The woman was the passenger in a Nissan Skyline struck by a Ford Falcon at the intersection of the Great Eastern Highway and Homestead Road in Mahogany Creek about 7.45pm on Friday.
She died at Midland Hospital.
The Skyline split in two when it was hit by the Falcon, smashing debris all over the road.
Witness John Walker told the Nine Network: “It looked like a bomb had gone off”.
The Nine Network named the couple as 21-year-old Maddie Morgan and boyfriend Jack Bryant, 23, and said they were expecting their first child in less than three weeks.
Mr Bryant suffered serious head and chest injuries and is fighting for life in Royal Perth Hospital.
One friend told Nine the tragedy was “so unfair”.
“He was happy to be a dad, and happy to be starting this new stage in his life,” he said.
The 23-year-old male driver of the Ford Falcon and a 19-year-old passenger were taken to Royal Perth Hospital with serious injuries, while another teenage passenger was uninjured.
Anyone with information, including dash cam footage, is asked to contact Crime Stoppers.