Like most mammals, female dogs undergo estrus, or a heat cycle, rather than experiencing menstruation. This usually occurs for the first time between the first nine and 12 months of the dog's life and reoccurs either biannually or annually from that point.
The first stage of the heat cycle is called proestrus. This usually lasts nine days, although this can vary by breed. In proestrus, the dog's vulva becomes swollen and the vagina emits a reddish-brown discharge. Estrogen levels rise, although mating does not occur yet. The second stage, estrus, follows and is typified by flirtatious behavior around male dogs and the ability to successfully become pregnant once ovulation occurs. Diestrus is the third stage and follows the same pattern whether the dog is pregnant or not. Anestrus, the final stage, lasts the longest and is the dog's way of preparing for another cycle.