Puberty is part of growing up, even for your dog. The proestrus stage is most like menstruation.
First, the Basics
Lady's not spayed, she'll experience a reproductive cycle. As a rule, smaller dogs tend to start their reproductive cycles at a year of age or earlier, while large breed dogs begin later -- as late as 24 months old. On the average, a dog will come into heat about every six months, but the interval varies from dog to dog. Some dogs experience an estrous cycle only once every year; others experience as many as three heat cycles a year. Anestrus is the longest phase of the cycle, when a dog is not receptive to mating; it lasts anywhere from 90 to 150 days, according to LSU vet school faculty member Dr. Bruce E. Eilts.
Coming Into Heat
Proestrus is the phase in the cycle when the dog "menstruates." This stage can last as briefly as three days or as long as 17 days. The amount of bleeding a dog experiences varies according to the dog; it is often the first thing an owner notices when the dog comes into heat. The discharge will change as Lady's cycle progresses, lightening to a watery pink or red-tinged discharge about a week or so into her cycle. You may see some gentleman callers at this point, as male dogs are able to pick up the scent of her pheromones and hormones during proestrus. Though she's sending out signals, Lady still isn't interested in suitors -- yet.
Looking for Love
Estrus is the next phase, lasting between three and 21 days. Lady may still be experiencing a thin, watery, straw-colored discharge while she's in estrus. At this point she's interested in male company and can become pregnant at any point of her estrus phase. She may seem more playful and may engage in tail flagging, or lifting her tail to the side. Diestrus follows estrus, whether or not Lady's pregnant. If she's mated, she'll spend the next 63 days carrying her pups and preparing for birth. If she didn't become pregnant, her body will behave as though it is pregnant with elevated levels of progesterone. When diestrus is complete and her hormones go back to normal, she'll enter anestrus again.
Spaying Takes Care of It All
There won't be any concerns over menstruation, pregnancy or unwanted suitors if Lady is spayed before she has her first heat. It's a myth that a dog should have a litter of puppies before she's sterilized. A litter of puppies won't improve her personality or health. If you have no plans to breed her, have Lady spayed before her first estrous cycle begins.