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.

What to Watch For


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:


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.