The victims were discovered at about 8 am in a bedroom in Hong Loc Commune, according to local officials. They were a 26-year-old woman, her two-day-old baby and her 23-year-old sister.
Police and emergency services were sent to the scene, where a charcoal brazier used for heating was found inside the room.
Relatives said the woman had recently given birth and had returned to her parents’ home to recover. With cold weather in the area, the family had burned charcoal indoors to keep the mother and newborn warm.
Authorities said the deaths were suspected to have been caused by carbon monoxide poisoning in an enclosed space. An investigation is under way to determine the exact cause.
Ha Tinh has experienced a cold spell over the past week, with temperatures between 17 and 22 degrees Celsius.