Canines typically menstruate twice each year, with most coming into heat every 6 months, although menstrual rates vary among breeds. Small dogs typically begin menstruating at earlier ages than female canines of larger breeds. Some smaller canines menstruate three times per year, while some female dogs of large breeds only menstruate once each year or every 18 months.
As with humans, dogs go through puberty as they grow and develop. Male and female dogs of all species develop reproductive capabilities during puberty. Depending on the breed of dog, the length of puberty varies, and commences at different ages. The estrus, or menstrual, cycle of most female canines begins when dogs reach the age of 6 months. Some smaller dogs, however, begin menstruating earlier. The first menstrual cycle experienced by dogs indicates sexual maturity. At that age, dogs conceive and produce offspring. As with humans, canine menstrual cycles may be irregular. This typically occurs when female dogs reach sexual maturity. Small dogs might take several months to experience regular cycles. For larger breeds, achieving regular estrus patterns takes up to 2 years. Estrus cycles, regardless of breed, generally last 2-3 weeks after onset. Signs of active estrus cycles include swelling of the vulva and discharge, which sometimes contains small amounts of blood.