Saying "they must be in heat to get pregnant" makes it sound like the cat is safe to go outside when they are in between heat cycles without having to worry about getting pregnant. If they are intact, they can get pregnant at any time if a willing male is in the vicinityCats are induced ovulators, but that doesn't mean they can get pregnant when they are not in heat! They must be in heat to get pregnant. However, it is possible to have a quiet heat cycle that the owner doesn't notice.