On average, a female dog that hasn't been neutered or spayed goes into heat twice a year. The duration of the heat cycle is between 12 and 21 days, although this varies between breeds and individual dogs and can last up to four weeks.
A dog usually experiences its first heat cycle between the ages of 6 and 12 months and can become pregnant at this point. The heat, or canine estrus, cycle has four stages; proestrus, estrus, metestrus and anestrus. Male dogs are attracted to the female during the proestrus stage, but the female isn't receptive until the estrus stage, when bleeding occurs.