During proestrus, the bitch becomes attractive to male dogs, but is still not receptive to breeding, although she may become more playful. A serosanguinous to hemorrhagic vulvar discharge of uterine origin is present, and the vulva is mildly enlarged. Vaginal cytology show a progressive shift from small parabasal cells to small and large intermediate cells, superficial-intermediate cells, and finally superficial (cornified) epithelial cells, reflecting the degree of estrogen influence. Red blood cells are usually, but not invariably present. The vaginal mucosal folds appear edematous, pink and round. FSH and LH levels are low during most of proestrus, rising during the preovulatory surge. Estrogen rises from basal anestrus levels (2-10 pg/ml) to peak levels (50-100 pg/ml) at late proestrus, while progesterone remains basal (<1 ng/ml) until rising at the LH surge (2-4 ng/ml). Proestrus lasts from 3 days to 3 weeks, with a 9 day average. The follicular phase of the ovarian cycle coincides with proestrus and very early estrus.

During estrus, the normal bitch displays receptive or passive behavior, enabling breeding. This behavior correlates with decreasing estrogen levels and increasing progesterone levels. Serosanguinous to hemorrhagic vulvar discharge may diminish to variable degrees. Vulvar edema tends to be maximal. Vaginal cytology remains predominated by superficial cells; red blood cells tend to decrease but may persist throughout. Vaginal mucosal folds become progressively wrinkled, or crenulated in conjunction with ovulation and oocyte maturation. Estrogen levels reduce markedly after the LH peak to variable levels, while progesterone levels steadily increase (usually 4-10 ng/ml at ovulation), marking the luteal phase of the ovarian cycle. Estrus lasts 3 days to 3 weeks, with an average of 9 days. Estrous behavior may precede or follow the LH surge, its duration is variable, and may not coincide precisely with the fertile period. Ovulation of primary oocytes occurs 2 days after the LH surge; oocyte maturation occurs 2-3 days later, the lifespan of secondary oocytes is 2-3 days.

During diestrus, the normal bitch becomes refractory to breeding, with diminishing attraction of male dogs. Vulvar discharge diminishes and edema slowly resolves. Vaginal cytology is abruptly altered by the reappearance of parabasal epithelial cells and frequently neutrophils. The appearance of vaginal mucosal folds becomes flattened and flaccid. Estrogen levels are variably low, and progesterone levels steadily rise to a peak of 15-80 ng/ml, before progressively declining in late diestrus. Progesterone secretion is dependent on both pituitary LH and prolactin secretion. Proliferation of the endometrium and quiescence of the myometrium occur under the influence of elevated progesterone levels. Diestrus usually lasts 2-3 months in the absence of pregnancy. Parturition terminates pregnancy 64-66 days after the LH peak. Prolactin levels increase in a reciprocal fashion to falling progesterone levels at the termination of diestrus or gestation, reaching much higher levels in the pregnant state. Mammary ductal and glandular tissues increase in response to prolactin levels.