The frequency and length of time a dog goes into heat varies by breed, but is approximately twice a year for about 18 days. Although the heat lasts this long, the female dog is typically only open to mating for about nine of these days.
Female dogs reach their first heat, also known as estrus, between six and 24 months, so veterinarians recommend spaying before this time. During the heat, a female dog may behave differently, including being alert and presenting hindquarters to male dogs, due to hormonal changes. During this time they also emit a discharge for which breeders or owners may use special diapers on the dog to prevent messes.