The canine estrous cycle, or what we think of as "being in heat," has four phases called proestrus, estrus, diestrus, and anestrus. 

Proestrus

Proestrus is normally thought of as the beginning of the estrous cycle. During proestrus, estrogen is produced by the ovaries, which results in a bloody discharge from the vulva and swelling of the vulva. Proestrus can last a few days to several weeks but, on average, lasts about 10-14 days. During proestrus, females may attract males, but they are not receptive to breeding.

Estrus

At the end of proestrus, there is a sudden, brief rise in luteinizing hormone (LH). This rise is referred to as the LH surge. The LH hormone works with the ovaries to cause the follicles to release their eggs. The LH surge marks the beginning of estrus, which is the next phase of the estrous cycle. Estrus is the phase in which females will allow themselves to be bred. Ovulation occurs during estrus, and if she is bred, the eggs are also fertilized during estrus. Estrus is characterized hormonally by a rise in progesterone and a decline in estrogen. Estrus lasts about one week. 

Proestrus and estrus together are referred to as the “heat cycle.” The first heat cycle occurs when females reach about 2/3 of their adult size (six to 18 months of age), with some notable differences. Sighthounds like greyhounds, borzoi, and Afghan hounds may not have their first heat cycle until they are well over two years old. Any female will have heat cycles throughout their life unless they are spayed. During a spay, the ovaries are removed. Without the ovaries, they do not produce estrogen or progesterone and do not come into heat anymore. Most females are spayed to prevent heat cycles and pregnancies as well as to prevent common reproductive diseases like pyometra (pus in the uterus).

Diestrus

Diestrus follows estrus. Diestrus is characterized by high levels of progesterone whether or not she is pregnant. Elevated progesterone levels are necessary to maintain pregnancy, and high progesterone levels are responsible for some of the physical and behavioral changes seen during diestrus, including mammary gland enlargement, increased appetite, and milk production. Diestrus lasts two months if she is pregnant and can last a little longer (2.5 months) if she is not pregnant. The non-pregnant diestrus is also called a false pregnancy or pseudopregnancy. At the end of diestrus, progesterone levels return to baseline.

Anestrus

The final phase of the estrous cycle is anestrus. If your dog gives birth to puppies, she will produce milk and rear her puppies during anestrus. This is the stage in which the uterus recovers and repairs itself in preparation for the next heat cycle. Anestrus is characterized by low progesterone and low estrogen. Anestrus can last for two to 10 months, with an average of four months. 

The normal estrous cycle repeats every six months with a range from four to 12 months. Some breeds, including Basenji and Tibetan mastiff, are known for only having one heat cycle in the fall each year. 

Currently, there are three procedures most performed under the term “spay”. Your veterinarian may perform an "ovariohysterectomy" (referred to as OVH), which removes the ovaries, uterine body, and uterine horns. Another choice is ovariectomy (OVE), in which only the ovaries are removed. Your veterinarian may perform a hysterectomy, removing the uterus only and leaving the ovaries. Discuss with your veterinarian which option would be best for your dog, and why.

If you prefer that your female dog does not have puppies, speak to your veterinarian about the best age to spay her. If you do want your dog to have puppies, ask your veterinarian what medical care to expect and when. If your dog is a purebred, breed clubs can help guide you about what to expect. The more you know, the easier it will be for everyone.