A neighbor pulled the fire alarm when he saw smoke coming from a corner unit of the ninth floor in the 28-story building at 6 p.m. Tuesday, the Toronto Star reported.
Inside, firefighters found three women, one of them eight months pregnant, and her 4-year-old son with no vital signs together in a bedroom, Fire Chief Bill Stewart said.
Stewart said the apartment was being treated as a crime scene as there was no apparent reason the four didn't try to escape and no immediate indication of what started the fire.
"Six o'clock at night -- it does not make sense. People would be up and around," Stewart told reporters.
The surviving two women were listed in critical condition Wednesday in a Toronto General Hospital hyperbaric chamber used to treat carbon monoxide poisoning, the report said.
The fire was contained to the one unit in the concrete building but was very intense, firefighters said.