Staff Sgt. Rick Tuza announces on Friday that a Calgary mother has been charged with killing her baby in 2004. ((CBC))

Police in Calgary have charged a woman with killing her baby six years ago after the death was originally ruled as being due to natural causes.

Stacey Joy Bourdeaux, 33, is charged with second-degree murder. Her 10-month-old son was found in his crib, not breathing, on Dec. 27, 2004.

Investigators reopened the file after Bourdeaux took her five-year-old boy to Alberta Children's Hospital on May 27.

The child was having trouble breathing and was struggling to stand, symptoms consistent with some form of trauma, police said.

Bourdeaux was charged a few days later with attempted murder, choking with intent and failing to provide the necessities of life.

Police then began an extensive investigation into the family's history, prompting officials to reclassify the 2004 death as a homicide.

When the child's body was found, emergency crews were called to the home in the 4900 block of 35 Avenue S.W. and took the child to hospital, where he later died.

The cause of death was ruled as natural causes based on information available at the time. There was no criminal probe.

But during the current investigation, Bourdeaux confessed to killing her baby in 2004, staff Sgt. Rick Tuza of the Calgary Police Service told reporters at a news conference on Friday.

"There would have been nothing to draw police to that initial investigation" had the second assault not happened, said Tuza.

Bourdeaux is being held at the Calgary Remand Centre and has been co-operative with investigators, he said.

The father of the two children died in an accident earlier this year, police said.

In 2005, Bourdeaux was taken to hospital "for her own well-being," Tuza said.

Police did not reveal any more details about the extent of the five-year-old's injuries or whether he remains in hospital.