An 18-year-old woman accused of fatally assaulting her baby at the family's Capel home in WA's South West has had her charge upgraded to murder.

Cassandra Rose Doohan was originally charged with causing grievous bodily harm to her four-month-old daughter Anastasia Hand, who died in May this year.

An earlier court hearing was told Ms Doohan had been left alone with the baby for about 15 minutes and when her partner returned from having a shower, and picked up the child, she was unresponsive.

The child died two days later in Princess Margaret Hospital.

The court heard doctors found she had suffered a catastrophic brain injury that was not accidental.

At the time, Ms Doohan was refused bail with a magistrate describing the case as serious and tragic.

Today, during another brief court appearance, the grievous bodily harm charge against Ms Doohan was discontinued and substituted with a charge of murder.

Her lawyer, Seamus Rafferty, indicated his client would be pleading not guilty.

Ms Doohan was remanded in custody and will face court again in January.