You dog experiences an estrous cycle, commonly referred to being in heat, but Shelby won't have a menstrual cycle like a woman does. While there is a bloody discharge during part of her reproductive cycle, it's not a period. The only way to keep your dog from experiencing estrous and all that comes with it is to spay her.
Four Parts to the Cycle
Shelby's estrous cycle consists of four stages: proestrus, estrus, diestrus and anestrus. Proestrus, which lasts an average of nine days, is the time when she'll first start to bleed and attract suitors. Since your dog bleeds during her cycle, it's easy to consider it a "period," however it's different from human menstruation. Her blood comes from her vaginal walls; a human's menstrual flow is the shedding of her uterine wall. Shelby will have plenty of male suitors at this point in her reproductive cycle, but she won't be interested in them and can't get pregnant.
Ready to Reproduce
She'll change her mind when she's in estrus, which lasts around five to nine days. Shelby will still have a discharge at this time, ranging from bloody to straw-colored. Her body will know it's time to get pregnant, and she'll become receptive to suitors.
Pregnant -- or Not
The next part of her cycle is diestrus. If she's pregnant, diestrus lasts 56 to 58 days and her body will transform as she begins to grow her pups and prepares to give birth. If she's not pregnant, her body will still be in diestrus, and she may respond as though she is pregnant because her body is producing the same hormones during this phase. She may produce milk, nest and choose toys to "mother" during this time. This false pregnancy can last for up to several weeks. When the diestrus stage is over, Shelby will enter anestrus for four or five months, where she's not interested in males; they're not drawn to her and she can't get pregnant.
Spay is the Only Way
Generally, dogs go into heat every six to eight months and experience their first heat cycle around six months of age. If Shelby's a small breed dog, she may start as young as four months; if she's a big girl, she may be a year or older before she reaches puberty. There is no magic pill to keep her from going through her cycle or experiencing the bloody discharge that accompanies it. The only way to stop a dog's cycle is to spay her. If you are having a tough time dealing with the bleeding that comes with her cycle, you can find doggy diapers in your local pet store or online.
Keeping Heat Away
Spaying not only keeps her discharge away; it cuts down on animal population. Your vet will tell you when it's best for Shelby to have her surgery. Generally, it's recommended to spay a dog before she has her first heat. If she goes into heat and experiences a false pregnancy, wait until her false pregnancy is over to spay her; spaying her during this time can actually prolong the experience.