Canine Estrous Cycle and Estrous Monitoring

One must grasp an understanding of the normal reproductive cycle in the bitch to efficiently breed, manage pregnancy and parturition, and diagnose urogenital diseases. Estrous designates the entire reproductive cycle of the bitch from standing heat (breeding) through 3 non-breeding stages back to standing heat. The entire estrous cycle averages about 7 months.

Estrus—standing heat—last a mean of 9 days. Unique to the dog, the hormones responsible for the behavioral signs of estrus are a decreasing level of estrogen and an increasing level of progesterone (P4)—both produced by the follicles. Ovulation occurs in mid- estrus following a 24-48 hour surge in luteinizing hormone (LH).
Estrus is followed by diestrus (or metestrus)—the pregnancy period. The bitch is no longer receptive to the male. The predominant hormone is P4 produced by the corpus luteum (CL). During pregnancy, diestrus lasts 57 days; cut short by fetal induction of parturition. During non-pregnancy, diestrus may last over 75 days as the canine uterus does not produce prostaglandins to induce luteolysis.
Following diestrus, the bitch enters a period of apparent reproductive quiescence termed anestrus. Reproductive hormones are at a baseline level for approximately 4 months. The mechanism regulating the end of anestrus and return to heat is not fully understood. It is evident however that dopamine plays a role as dopamine agonists (eg. bromocriptine) can induce an early return to heat. The role of prolactin is uncertain. Unlike the horse, however, where elevated prolactin concentrations are associated with an end to anestrus, elevated dopamine and subsequently decreased prolactin concentrations are associated with a return to heat in the dog.
The period when the bitch begins to be receptive again, but is not willing to stand for the male is proestrus. It last a mean of 9 days and is the result of rising concentrations of Estrogen produced by the developing follicles.  [...]