Menstruation is feature of group of animals called primates which also includes humans. In dog menstruation or periods don't occur. Instead, dogs have heat cycle. This heat period can start as early as 4 months and as late as more than one year of age in dogs. Average age of dogs at the start of heat cycles is 6-8 months. Medically, it is not recommended to breed dogs after the first heat cycle because they are not physically mature to afford stress of pregnancy and delivery. They should be used for breeding after 2-3 heat cycles.