Reproductive cycle of the bitch

Proestrus

Average duration: 9 days; range = 3-17 days

Swelling of the vulva, the external tissue of the vaginal opening, and bloody discharge marks the beginning of the proestrus stage, also known as the follicular stage. During proestrus, the ovarian follicles, each containing ova, increase in size. Increasing amounts of estrogen hormone, secreted by the ovarian follicles, cause the cells of the vaginal walls to take-on a distinctive shape, a process known as cornification. Both the level of estrogen and vaginal cornification are useful indicators of proestrus.

Estrus

Average duration: 9 days; range =3-21 days

Receptivity to mating marks the beginning of the estrus stage. Physiologically, estrus coincides with the predominant presence of cornified vaginal epithelial cells and an increase in serum progesterone levels to 2 ng/ml. Ovulation usually occurs 2 days following this increase in progesterone and hence, monitoring the levels of progesterone is an excellent indicator for timing breeding.

Diestrus

Average duration: 2 months

Approximately 6 days after ovulation, the cornified vaginal epithelial cells will revert to a non-cornified state. This condition marks the beginning of diestrus. This stage ends when progesterone levels fall to less than 1 ng/ml just prior to whelping in the pregnant bitch or approximately 2 months after ovulation in the nonpregnant bitch.

Anestrus

Average duration: 4-4.5 months

The beginning of this stage is marked by the drop in serum progesterone levels to less than 1 ng/ml. The beginning of proestrual bleeding marks the end of this stage. Duration of anestrus is quite variable among bitches and may be governed by both genetic and environmental variables.