The dog heat cycle can vary dramatically. Dogs go into heat on average twice a year, but some dogs may go into heat three times and many larger breeds only once annually. The cycle lasts an average of 12 to 21 days but can be as short as a few days or last as long as four weeks.
A female's first heat cycle will usually occur sometime between 6 months to a year, although it may occur as early as 5 months in smaller breeds and as late as 14 months in larger breeds. Females can get pregnant during the first heat, though it is not advisable. Most females have not been tested for breed-specific health issues by this time, and most behavioral or health issues aren't going to occur until the female is more mature. Pregnancy complications are also more likely at this young age.
The estrus cycle can be divided into four stages:
Proestrus
This stage may last anywhere from 4 to 20 days. You may see some blood or vaginal discharge. Your female will be unwilling to mate, but males will be very interested. Estrogen production increases during this time as the uterus enlarges and prepares for fertilization. Increase in estrogen causes the vaginal walls to take on a distinctive shape, known as cornification. Measuring levels of estrogen as well as checking for cornification can be good indicators that the heat cycle has begun.
Estrus
This stage lasts 5 to 13 days, and mating will occur at this time. The vulva is swollen, and there may be yellowish discharge. Increased estrogen production gives way to increased progesterone production as the body prepares to nourish the eggs once fertilized. Measuring progesterone can be a good indicator that your female is ready to mate, but behavioral changes will also be easy to spot. Females will now be seeking out the males to encourage mating by giving clear signals such as backing toward the male and wagging her tail in wide swoops to alert males to her scent.
Diestrus
This is the period after mating when, again, the females are not willing to mate. If your female is pregnant, the pregnancy will last 60 to 64 days. Otherwise, the length of this stage is 60 to 90 days. During this stage, progesterone, which is produced to nourish the eggs and help them grow into fetuses, may cause "false pregnancy" symptoms such as nesting behavior and milking, even if the female is not pregnant. This phase ends when progesterone levels have returned to normal either after whelping new puppies or after an approximately two month period for non-pregnant females.
Anestrus
The 2 to 3 month period of inactivity in between heat cycles.
If you aren't interested in breeding your dog, many veterinarians recommend spaying your dog before her first heat cycle, which offers the greatest protection against reproductive-related diseases later in life.