Female dogs usually experience their first heat cycle at about the age of 6 months, although that initial estrous cycle often occurs considerably later in certain breeds. If not spayed, the dog goes into heat an average of twice annually. A dog's cycle consists of four distinct stages, but she can only get pregnant during the estrus period. Her entire heat cycle lasts between two and three weeks.
During proestrus, the dog's vulva swells and she starts bleeding. The amount of bleeding depends on the individual. Some female dogs bleed so little it's hardly noticeable, while others bleed heavily. It's a good idea to take your dog for potty breaks on a leash during this time rather than let her out in even a fenced yard without supervision. Even though she's not ready for mating yet and won't cooperate, intact male dogs are interested. Pheromones in her urine leaves messages for them.
The estrus stage starts about a week to 10 days after proestrus. At this point, her discharge changes from bloody to watery. She's ready and willing to breed. The estrus period lasts between five and 10 days. The American Kennel Club website warns that while the average dog might ovulate on day 11 of her cycle, so that her best opportunities for pregnancy occur on days 9 through 13, many dogs aren't average. If you're planning to breed your dog, get your veterinarian involved beforehand to ensure she's healthy enough for reproduction. Your vet also can perform blood tests to reveal when your dog's progesterone levels increase -- a sign that she's ovulating.
If your dog breeds and become pregnant, she'll give birth to her puppies in approximately 63 days. The diestrus period involves pregnancy in the bred dog, or the two-month period following the end of the estrus cycle if the dog wasn't bred or the mating didn't "take." If you are breeding your dog, when she first decides she wants nothing to do with the male after successful copulations, that usually indicates the end of the estrus and the start of the diestrus period.
During the anestrus cycle, female dogs show no interest in mating. Intact male dogs shouldn't show any interest in them, either. This period between estrous cycles ranges between 50 and 80 days, depending on the individual dog.