Dog Heat Prebent Breeding. Calcium sulfate, thymol, camphor, and eucalyptus directions: The female dogs most commonly are sexually maturing at 8 months to 1 year of age, and this is manifested by the onset of the first reproductive cycle.

Dogs can go into heat as young as four months in smaller breeds, but averages about six months old. When a female dog is in heat, she is receptive to mating and can become pregnant; However, because your dog is most fertile before you may actually realize she is in heat, you need to work to prevent breeding before you realize your dog is fertile and cycling through her season.