Phase	Duration	Physiological Events	Observable Signs	Endocrine	Behavior
Proestrus	6 to 11 days, with an average of 9 days. However, variations within what is considered normal can be exceptional: as brief as 2 or 3 days to as prolonged as 25 days).	This is the initial phase of the estrus cycle. Hormones produced by the ovaries trigger a swelling of the womb which then creates a lining in preparation for possible pregnancy.	The vulva swells, stiffens, and emits a bloody discharge. The amount of discharge varies widely from dog to dog.	Estrogen levels rise gradually and peak 1 to 2 days prior to estrus.	During this period, the female draws the male in by emitting pheromones. Behavior includes an increase in playful, teasing activity but active discouragement of any mounting attempt by a male. This may involve growling, baring the teeth, and snapping. The bitch may also keep her tail tight between her rear legs.
Estrus	9 to 18 Days	Ovulation can occur during this phase, usually one to two days into Estrus.	The vulva becomes softer, and the bloody discharge may change to a clearer color.	Estrogen levels decline; progesterone levels rise. Amounts of serum progesterone exceeding 1 nGm for 4 days are generally indicative of Estrus.	This phase is often called "standing heat," because the bitch will often stand still and present her posterior end to the male. This is seen as her raising her tail and shifting it to one side, displaying her genitalia. Generally, The more flagrant this behavior, the more likely it is that she is ready to breed.
Parturition	About 63 Days	Gestation	Swelling of nipples and abdomen	Progesterone is predominant until whelping	Decreased activity
Anestrus	Three to five months	The system rests during this period.	No signs present.	Estrogen and progesterone at anestrus levels	Typical social behavior