Police hope to decide next week whether to lay a third murder charge against a teenager accused of killing Brisbane parents-to-be Kate Leadbetter and Matty Field, as well as their unborn child.

The autopsy on Ms Leadbetter, 31, to confirm the details of her pregnancy was completed last week after her and her partner Matty Field, 37, died in Alexandra Hills on Tuesday.

The advice centres on when an unborn child is considered a life and whether prosecutors could argue their case in court if they laid a third murder charge against the accused 17-year-old male.

Senior police said the circumstances are complicated, but once the final medical and legal advice has come through, they hope to make a decision soon after. This is hoped to be early next week.