Bitches become sexually mature when they have their first heat period, usually between 6 and 12 months of age, although there is considerable variation. Toy breeds become sexually mature several months earlier than giant breeds. Sexual maturity does not correspond to physical maturity – the attainment of adult height and weigh – which generally happens at about 14 months of age in medium-size dogs.
Ovarian activity begins to decline after 6 years of age and ceases in most bitches at around 10 years of age. By 8 years of age it is marginal. Most bitches are not used for breeding after 7 to 8 years of age.
Females generally come into heat every five to nine months. (Basenjis are noted for only having a heat once a year.) The heat cycle is specific for each individual. Unlike some other animals, the heat cycle in dogs is not controlled by outside influences such as the number of hours of daylight.
The estrous, or heat, cycle is divided into four stages, each corresponding to the principal hormonal influence governing that stage. The combination of proestrus and estrus, which breeders refer to as the heat period, lasts an average of 21 days.
Proestrus
The initial stage of the heat cycle lasts an average of nine days (the range is 3 to 17 days). The first sign is a bloody discharge from the vulva. In early proestrus, the discharge may be light pink to yellow. If you are not sure about the discharge, wipe a tissue across the vulva. If you see a pinkish color, the bitch is in heat. Along with a discharge, there is firm swelling of the vulva.
During proestrus the female makes chemical substances called pheromones. These substances attract males, although many experienced stud dogs will not show strong sexual interest in a bitch until she is in estrus.
During the first four to five days of proestrus, the female shows no interest in breeding. If mating is attempted, she will sit down, jump away, growl, or snap at the male. A few days before estrus, the next phase, she may be willing to let the male mount her but she won’t stand for breeding. Proestrus ends when the bitch becomes receptive to the male.
Estrus
The second phase of the heat cycle is called estrus, or standing heat. This is when the female is willing to breed. Estrus lasts seven to nine days (the range is 2 to 20 days). It ends when the bitch refuses to stand for the stud.
As estrus begins, the vulva softens and becomes pliable in preparation for intromission. The discharge becomes watermelon colored or pink. At this time the female begins to flirt with the male, raises her tail and flags it to the side, lifts her pelvis, and presents her vulva when touched in the rear.
Ovulation usually occurs on the second day of estrus, or about the 12th day of the heat cycle as measured from the first day of proestrus. Keep in mind that ovulation may occur sooner or later than expected, owing to variations in the lengths of proestrus and estrus.
A microscopic examination of vaginal secretions (a test called vaginal cytology) helps pinpoint the beginning of estrus. There are characteristic changes in the appearance of vaginal cells that tell your veterinarian when the bitch is in estrus, and later, when she is in diestrus.
Vaginal cytology, however, is not an accurate method of predicting when ovulation will take place. A more accurate method is to measure serum progesterone levels. The serum progesterone level remains low during early proestrus (less than 2 nanograms per milliliter). As ovulation approaches, the serum progesterone begins to rise. The reason progesterone rises is that it is stimulated to do so by the rise in luteinizing hormone (LH), as shown on the chart on this page. Note that LH peaks rapidly and drops precipitously. The LH surge triggers ovulation, which occurs two days after the peak. As LH peaks, the serum progesterone rises above 2 ng/ml. Thus, when the progesterone measures above 2 ng/ml, one can predict that ovulation will occur within the next 24 to 48 hours.
Progesterone levels can be measured by your veterinarian using a rapid inhouse assay kit (ELISA). The test is not done for routine uncomplicated breedings. A radioimmunoassay test also is available, but blood must be sent to a reference laboratory. LH levels can also be measured, but should be studied in combination with progesterone levels to be sure you didn’t miss the peak surge.
Timing the breeding with ovulation increases the probability of breeding at the most fertile time in the estrous cycle—72 hours after ovulation. This is particularly important in planning artificial insemination and in attempting to impregnate bitches with infertility related to abnormal heat cycles or failure to show heat.
Diestrus
This third stage in the estrous cycle, also called the luteal phase, begins when the female refuses to stand for breeding. The male also loses interest at this time. Diestrus can be confirmed by vaginal cytology. It lasts about 60 days, and then merges with anestrus. If the bitch becomes pregnant, diestrus lasts 56 to 58 days and is followed by whelping.
Anestrus
The fourth phase of the estrous cycle is a period of sexual quiescence during which the progesterone-stimulated endometrium undergoes repair. Progesterone levels are low, indicating little if any ovarian activity. The length of anestrus varies, lasting on average 130 to 150 days. Anestrus is followed by the beginning of a new heat cycle.