A woman who killed her 18-year-old son, stabbing him as they argued at a Perth bus station, has pleaded guilty to manslaughter after fighting a murder charge.

Tanya Mynette Ugle, 37, was thrown off a bus for disorderly behaviour moments before the attack on Jamahl Jordan Ugle in Fremantle on the afternoon of November 23, 2016.

Earlier this week, Ugle stood trial before a judge-alone in the Supreme Court of WA, which heard heard the pair were seen yelling at each other before she walked away, leaving behind the bleeding teen.

A transit officer testified in a written statement that a member of the public alerted him to the injured man, saying “he needs help”.

He said Jamahl was breathing frantically, holding his hand over the puncture wound and saying “it hurts” before slumping on a bench while Transperth staff applied first aid.

The transit officer and a colleague chased Ugle to a nearby shopping centre, where she bought milk, dropping the container when she saw they had caught up with her.

She was put in handcuffs until police arrived.

One of the officers said she was swearing and screaming incoherently as they tried to inform her of her rights, was “clearly drug affected” and claimed her son “pushed me … beats me”.

A kitchen knife with a bent end was found tucked into her pants.

Family members who had gathered around shouted that she was pregnant, and another transit officer said she had an “early-stage baby bump”, estimating she was about half-way through her term.

The Director of Public Prosecutions accepted Ugle’s guilty plea to manslaughter on Tuesday.

She faces a sentencing hearing on Friday.

© AAP 2018