The Heat Cycle of Dogs

The Heat Cycle of Dogs

Estrus in the bitch (female dog) is defined as the time during the reproductive cycle when she displays interest in mating and has probably ovulated or is about to ovulate. Estrus begins when the bitch allows the male to mount and breed, and ends when her receptive behavior ceases.

General Causes

Bitches reach sexual maturity (puberty) between 4 to 18 months of age, at which time they experience their first estrus. There is a tremendous variability in the maturation age between breeds, and even within a breed of dog.

Several major phases compose the estrous cycle. Variations in the level of normal circulating hormones contribute to these different phases.

  • Proestrus – the period that precedes estrus, when males are attracted to non-receptive females. Proestrus generally lasts for approximately nine days.
  • Estrus – the period of breeding lasting approximately five to nine days; together with proestrus in known as the time of being "in heat".
  • Diestrus – the period following mating. Diestrus lasts approximately 56 to 58 days in the pregnant bitch, and 60 to 100 days in the non-pregnant bitch.
  • Anestrus – the period of reproductive quiescence. The bitch has no attraction to or from the male. Anestrus generally lasts four to five months.

    What to Watch For

  • Small amounts of bloody vaginal discharge are associated with proestrus. Initially, the vulvar lips swell and become soft and pliable.
  • Restlessness
  • Frequent urination
  • Ability of the female to attract males from long distances away
  • Receptivity to the male


    Vaginal cytology is a very helpful tool in documenting estrus in the dog, and measurement of serum progesterone helps to determine the day of ovulation in the bitch. If the estrous cycle appears to be abnormal, or the dog is believed to be infertile, then additional diagnostic tests may be recommended. These include:

  • A thorough history and physical examination
  • Baseline blood tests to include a complete blood count (CBC), biochemical profile, and urinalysis as a broad general health screen
  • Measurement of other hormones that affect the reproductive cycle, such as thyroid assays
  • Blood test for brucellosis, a bacterial infection of the reproductive track
  • Screening thoracic and abdominal radiographs
  • Abdominal ultrasonography


    Estrus is a normal physiologic function of the intact female dog. Estrus can be prevented or eliminated by spaying the dog. The medical term for spaying is ovariohysterectomy, which means that both the ovaries and uterus are surgically removed. All signs of estrus cease within a few days of spaying the dog.

    Home Care

    It is very important to be aware of when your female dog is in estrus. It is common for a bitch in estrus to attract males from all over the neighborhood. Inappropriate mating may occur, and preventative measures should be taken to keep the dog inside confined or constantly supervised by someone when outside.

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