Estrus in the bitch usually occurs twice a year, in the spring and again in the fall, although it may occur in any month of the year and may vary in occurrence from year to year. Sokolowski et al. (1977) report that Basset Hounds and Cocker Spaniels have mean interestrous intervals of approximately 5 months, whereas the German Shepherd Dog has the shortest interestrous interval, 149 + 28.5 days. The mean occurrence of estrus for the German Shepherd Dog is 2.4 times per year and for other breeds it is 1.5 times per year. Neither natural nor artificial light has any effect on estrus. Periods of sexual receptivity in the bitch can be recognized by behavioral patterns: lordosis, presenting to the stud, excitement of the stud, cytologic changes in the vaginal epithelium (blood cells and cornification as seen in a smear), visual changes of the vaginal epithelium as seen with an endoscope (), and more recently by hormone levels, particularly luteinizing hormone (LH) in the circulating blood (). The changes through which the nonpregnant uterus passes were designated by Heape (1900) as proestrum, estrum, metestrum, and anestrum. These terms were used by Evans and Cole (1931), Griffiths and Amoroso (1939), and other early investigators of the dog’s estrous cycle. Later workers suggested that the phase of estrus during which the reproductive organs are mainly under the influence of progesterone should be designated as diestrus, leaving the term metestrus for the short transition stage between estrus and diestrus when the corpora lutea are becoming functional. Investigations by Hoist and Phemister (1974), Concannon et al. (1975), and Concannon (1991) have facilitated a redefinition of the estrous cycle.
Proestrus is a period of increasing levels of estrogen, the discharge of serosanguineous fluid, vaginal cornification, and swelling of the vulva. Pheromones attractive to the male are secreted toward the end of the period, which may be as brief as 3 days or as long as 3 weeks, with a mean of approximately 9 days. During most of this period the bitch will not allow the stud to mount. She may turn and growl, sit down, or lie on her side.
Estrus is a period of heightened sexual activity in response to a decline in estrogen facilitated by a rise in progesterone. Estrous behavior includes standing firmly for the male to mount, deviation of the tail, and lordosis. Concannon (1991) has shown () that the onset of estrus most often occurs within a day or two of the preovulatory LH surge. However, it may occur 4 days before or 6 days after the LH surge. Estrus may be as brief as 3 days or last for several weeks, although the average length is 9 days.
Some breeders prefer to mate their dogs toward the end of the estrous period to ensure that a maximum number of ova will have been ovulated but this does not appear to be necessary. The embryos and fetuses used for description in site of this text were collected by Evans from more than 40 Beagle bitches bred to one stud 24 hours after they would first stand firmly and accept the stud. The conception rate for these matings early in estrus resulted in litters that averaged 6.7 pups per litter, which indicates that most ovulations had already occurred or sperm remained viable until the ovocytes were ovulated. It is known that dog sperm can live for several days in the female reproductive tract (average 2 or 3 days) and that a mature ovocyte can also live for several days (average 3 or 4 days). The average time of ovulation according to Concannon and Lein (1989) is 2 days after the LH surge. They found the peak period of fertility for natural matings ranged from 1 day before the preovulatory LH surge (= day 0) until day 5 or 6 after the LH surge. Based on their studies it can be said that breeding twice, between days 0 and 4, is probably best to ensure conception. This is probably what the normal bitch does if permitted. The length of gestation in the dog as timed by Concannon, using the day of the LH surge as day 0, results in gestation lengths of 64, 65, or 66 days, although the interval from mating to parturition may vary from 56 to 69 days. Evans (1974) considered day 0 to be 24 hours after first acceptance by the bitch, which resulted in an average gestation length of 60 to 63 days in his Beagles.
Metestrus is now synonymous with diestrus. It is a period without sharp boundaries when the corporea lutea are becoming functional and progesterone dominant. As this period begins the bitch may or may not accept the stud and there is an increase in the number of noncornified cells from the deeper layers of the vaginal epithelium. Vulval tumescence decreases but mammary development increases. Hoist and Phemister (1974) described the onset of diestrus with regard to the characteristics of the vaginal epithelium and backdated from these signs to show correlations with the early stages of development. Diestrus minus 1 day was the stage of a two-cell cleavage; D minus 3 = fertilization; D minus 4 = formation of a secondary oocyte; D minus 5 = beginning of meiosis; D minus 6 = ovulation. They found that if the day of fertilization (diestrus minus 3 days) is counted as day 1 then the Beagle had a gestation period of 60 days and whelping took place on day 57 of diestrus.
Anestrus is a period of quiescence of the reproductive system with a fall in circulating hormone levels and an absence of sexual behavior. It lasts 2 to 10 months, with an average of 4 months. Some bitches are consistent in interval; others are not. Changes in sexual behavior correlated with hormone levels have been reported by Concannon et al. (1977), but now that it is practical to measure the level of LH in the bitch it is possible to use the LH peak as the most fixed point in the ovulatory cycle. Ovulation usually occurs 2 days after the LH peak and birth will take place about 65 days after the LH peak.
Concannon et al. (1983) in their early studies of gestation length in the dog, reported that in 290 dogs in which apparent gestation length was estimated as the interval from the day of first mating to the day of parturition ranged from 57 to 72 days and averaged 65.3 days. The interval from the day of the peak in LH to parturition was less variable and ranged from 64 to 66 days and averaged 65 days. Fertile single matings 3 days before the LH peak provided evidence that the potential postcoital fertile longevity of canine sperm is at least 6 days and this contributed, along with variability in the onset of estrus, to the observed variation in apparent gestation length in the dog. The limited range in the interval from the day of the preovulatory LH peak to the day of parturition (64, 65, or 66 days) demonstrates considerable regularity in the sequential events of gestation in the dog.
Raps (1948) made a study of the developmental changes in the dog ovary, from 2 days after birth to the sixth postnatal month. His data show that primordial ovocytes (ovogonia) surrounded by follicular epithelial cells are present at 4 days of age, that at 15 days true primary follicle formation with granulosa cell development occurs, and that antrum formation is not observable until 6 months of age. Ultrasonographic studies by Yeager and Concannon (1990) and by Yeager et al. (1992) have correlated the LH peak with the echographic appearance of the conceptus ().