A 12-year-old Perth girl, who gave birth earlier this month, was impregnated by her 14-year-old stepbrother.

In a shocking twist of events, it's been revealed the dad of the healthy child was living in the same house as the girl when she fell pregnant at the age of 11.

It's understood the girl had no idea she was pregnant until she went into labour at the family's Perth home.

She was later taken to King Edward Memorial Hospital where she gave birth to the healthy child.

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A Department of Communities spokesman said it was working closely with police and the Health Department to ensure the welfare of all three children.

"The department is unable to comment on situations like this in order to protect the privacy of the individuals and their families," a spokesman told The West Australian.

West Australian Police Commissioner Chris Dawson said the situation was centred around welfare.

"It's a very delicate, sensitive situation but we're primarily concerned about making sure the right support mechanisms are in place," he said.

"While there is a police investigation, it's primarily centred on the welfare."