The estrous cycle is the time period when a female dog is sexually attractive to a male dog and can become pregnant. It refers to the physiological and behavioral changes that female mammals go through in order to reproduce. In species with distinct estrous cycles, reproductive abilities follow distinct patterns with recurring periods of non-fertility and peak fertility; these cycles are characterized by fluctuations in hormone levels that initiate the changes in physical and behavioral traits.
The canine estrous cycle is divided into four distinct phases: proestrus, estrus, diestrus and anestrus. The length of the cycle can vary from 18 to 24 months, with the average being about 21 days.
Proestrus
The proestrus phase is the first stage of the estrous cycle and can last anywhere from two to nine days. This is the beginning of the fertility period and is characterized by the female dog beginning to show signs of being in heat. These signs can include swelling and darkening of the vulva, a bloody or pinkish discharge from the vulva, and increased urination. The female dog may also become more affectionate and playful, and may lift her tail and present herself to males.
Estrus
The estrus phase is the second stage of the estrous cycle and can last anywhere from four to 14 days. This is the peak fertility period and is when the female dog is most likely to become pregnant. The signs of estrus are similar to those of proestrus, but the discharge from the vulva is usually clearer and less bloody. The female dog may also be more restless and vocal during this phase.
Diestrus
The diestrus phase is the third stage of the estrous cycle and can last anywhere from two to 20 months. This is the period of greatest fertility and the time when the female dog is most likely to become pregnant. During diestrus, the corpus luteum, a gland that forms on the ovary after ovulation, produces the hormone progesterone. Progesterone helps to maintain the pregnancy if the female dog has become pregnant. If the female dog has not become pregnant, the corpus luteum will regress and progesterone levels will drop, leading to the onset of the next estrous cycle.
Anestrus
The anestrus phase is the fourth and final stage of the estrous cycle and can last anywhere from two to nine months. This is the period of least fertility and the time when the female dog is least likely to become pregnant. During anestrus, the female dog's reproductive system is relatively inactive. The vulva is small and pale, and there is no discharge from the vulva. The female dog may also be less interested in mating during this phase.