TORONTO -- A drop in core temperature due to exposure to severe cold can slow the metabolism so much that a person can appear to be dead, say doctors, who believe that was likely the case with a newborn girl who had been declared dead on the weekend but "came back" to life.

The infant was born on a Toronto sidewalk Sunday after her mother went into labour while walking to the hospital. Mother and child were rushed to Humber River Hospital for treatment but the baby had no apparent vital signs and was declared dead.

About 90 minutes later, two police officers standing by the infant while waiting for the coroner to arrive suddenly noticed that a sheet covering the tiny body was moving.

The newborn was alive.

The officer found a pulse and alerted doctors. The baby girl was transferred to the Hospital for Sick Children, where she was in fair condition Tuesday, meaning she is conscious and may have minor complications, but has a favourable outlook.

Dr. Jamie Hutchison, an intensive care physician at Sick Kids, said the newborn likely had hypothermia -- a condition in which the body is rapidly cooled, leading to a dramatic slowing of metabolism, which in some cases can mimic death.

"That would be the most likely reason," Hutchison, who was not involved in the newborn's care but conducts hypothermia research, said Tuesday.

"And that's because the body requires a certain temperature for metabolism to occur. So with each degree drop in temperature, the metabolism of every organ slows."

Whether hypothermia occurs in a child or adult, once their body temperature drops below a certain threshold, the person appears to be comatose, he said. And if they get cold enough, the heart beat will slow and weaken so much that they have no perceptible pulse.

"They're not dead, but they appear that way," Hutchison said.

The phenomenon has given rise to a saying in medical practice: "A patient isn't declared dead until they're warm and dead."

"That's what the mantra is now -- you don't give up until the patient's warm and declared dead," said Gary Sieck, an expert in hypothermia at the Mayo Clinic in Rochester, Minn.

In other words, medical practitioners shouldn't assume a patient exposed to frigid temperatures is dead because their body is cold.

There are many examples of people who had severe hypothermia and were resuscitated with CPR and techniques to gradually warm their blood and organs, Sieck said. Many eventually recovered, even after many hours of being in a state mimicking death.

Perhaps the most famous example in Canada is the case of Erika Nordby of Edmonton, dubbed the "Miracle Baby." In the middle of the night in February 2001, the then 13-month-old crawled outside of the house where she was sleeping with her mother. She was wearing only a diaper and T-shirt. The temperature was --24 C.

When she was found outside in a snowbank at least two hours later, Erika's body temperature had plunged to 16 C; her toes were frozen together and her heart had stopped. It took a medical team 90 minutes to get her heart beating again.

But three days later, though Erika was suffering from severe frostbite, there were no signs of other physical injury or the brain damage that's typical of having the oxygen supply off for a long period of time.

In some instances, hypothermia can be beneficial: it can be induced in patients who have had a stroke or heart attack in a bid to significantly lower the metabolism and limit damage to the brain.

"This is where the therapeutic effect of being cold comes in," said Hutchison. "Because the brain's metabolism is slowed ... it actually protects the brain from damage."

Therapeutic hypothermia, as it's called, is also used for patients who have suffered traumatic brain or spinal cord injury.

"When accidental hypothermia happens -- as in this case -- babies are much more resilient to hypothermia," ieck said of the Toronto baby, whose name has not been released. "But there have been examples of adults who have recovered after many hours of hypothermia."

He cited the case of Danish teenagers whose dragon boat capsized in 2011, throwing them all into the icy water of a fiord. Several of the teens were thought to be dead at the scene, but almost all recovered after specialized treatment in hospital, Sieck said.

It's critical that a person who becomes hypothermic from exposure --for example, after falling through the ice into a lake or river -- be given immediate CPR until paramedics arrive, he said.

"Just continuing CPR will provide adequate circulation to keep the tissues alive. It's so important to continue CPR, even though the patient may appear to be dead."