The female dog does not have a period but a heat, or estrus, cycle. During the proestrus stage of the heat cycle, the female dog bleeds, and male dogs become attracted to her.
The heat cycle of the female dog has four stages. The proestus stage lasts four to 20 days, and the female is unwilling to mate. In the estrus stage, the female dog has a yellowish discharge from her swollen vulva and is willing to mate; this stage lasts five to 13 days.
If the female gets pregnant, the metestrus stage begins, which lasts until she gives birth. If the female does not become pregnant, the metestrus stage lasts 60 to 90 days. Finally, the anestrus stage, the period of time when there is no sexual or hormonal activity, lasts two to three months.