Vancouver police Const. Jana McGuiness said using a Taser on a young mother was the last thing officers wanted to do. (CBC)

A police officer shocked a mentally disturbed teenage mother with a Taser to save the life of her critically ill infant, the Vancouver police say.

The incident happened on Monday at about noon when officers and social workers went to the 18-year-old father's home searching for the mother, 16, and child, police said in a statement issued late Thursday night.

Misha Peterson and her child had recently been reported missing from a foster home.

Social workers from the Ministry for Children and Family Development were concerned for the infant's health because the child was born with a life-threatening medical condition that required immediate attention, police said.

'You still could see the mark in her neck where she got Tasered twice. … You know, they could have hit the baby.'— Doreen Duncan, grandmother of the baby's father

Police found Peterson and child at the home and negotiated with her for three hours, but the mother refused to give up the child, they said.

Const. Jana McGuiness said using the stungun on Peterson was the last thing officers wanted to do.

"This was an emotionally tense situation … at no time would one want to separate a mother from her child … it's the last thing anyone would want to do … and it was no doubt a traumatic incident for the mother," McGuiness said on Friday.

But because the child needed immediate medical care and Peterson was holding the child in a way that caused officers to fear the baby could be smothered, an officer finally stepped in and stunned the woman with a Taser in her arm and upper back, McGuiness said.

Doreen Duncan, the grandmother of the infant's father, said she was shocked police would use a Taser on a girl holding a baby. (CBC)

The shock caused the mother to release her grip on the baby, and an officer then took the infant, which was transported to hospital by ambulance.

Peterson was apprehended under the Mental Health Act and taken to hospital. She was later released.

Police reported no injuries to the child or mother, and said no charges are expected, but the child remains in government care.

McGuinness said police wouldn't normally have publicly discussed the incident because it involves a juvenile mother and a baby with critical illness, but once Peterson went to the media, police felt they had to respond.

The grandmother of the baby's father, Doreen Duncan, says she can't believe police would use a Taser on a girl holding a baby.

"They said she was holding baby too tight. You still could see the mark in her neck where she got Tasered twice. What if they missed it? … You know, they could have hit baby. She had a baby in her arms," Duncan told CBC News on Friday morning.