Major Crime Squad detectives are investigating the death of a four-month-old baby girl in the state's South West.
The baby died at Princess Margaret Hospital on Sunday after being taken to Bunbury Regional Hospital two days earlier.
Nine News Perth reported the child was found unresponsive in her Capel home on May 26.
The baby's parents, an 18-year-old woman and a 21-year-old man, are assisting police with their enquiries.
They reportedly had to be sedated after learning of the baby's death.
No charges have been laid.
"The Major Crime Squad is investigating the circumstances surrounding the girl's death and is waiting for the results of a post mortem examination," police said.