A Montreal woman facing first-degree murder charges in the drowning death of her young son will present an insanity defence, her lawyer said.

Louise Desnoyers, 49, has been charged with first-degree murder in the August 2006 death of her 8-year-old son, Nicholas.

Police allege Desnoyers, a former grade school teacher, drove from Montreal to Lake Champlain, about 80 kilometres south, last summer and held her son's head under water until he died.

She's alleged to have tied Nicholas's body to a boat before attempting suicide by slashing her wrists and swallowing pills, police said.

Desnoyers, who is now being held in a psychiatric hospital, will plead insanity once her trial begins, her lawyer, Bob Katims, said during a court appearance Thursday.

As a legal defence, insanity can establish a person's mental condition at the time of the crime.

She has already been deemed competent to stand trial.

Desnoyers' trial was supposed to start last month, but her lawyer asked for more time to prepare his defence.

Katims said his team is trying to find Canadian medical records for the Montreal woman, who has been treated for mental-health issues in the past.

The trial is expected to get underway sometime next year.