Proestrus is the first stage of a dog’s heat cycle, marked by a bloody vaginal discharge, vulvar swelling, and rising estrogen levels. It lasts about 9 days on average, though it can range anywhere from 3 to 17 days depending on the individual dog. During this phase, your dog will attract attention from males but won’t yet be willing to mate.
What Happens During Proestrus
Proestrus kicks off the entire reproductive cycle. The ovaries begin producing estrogen, which steadily climbs from baseline levels and peaks one to two days before the next stage (estrus, or “standing heat”). This estrogen surge drives all the visible changes you’ll notice: the vulva becomes noticeably swollen, and a bloody vaginal discharge appears. The discharge can range from bright red early on to a lighter, more straw-colored fluid as the phase progresses.
Your dog’s body also starts releasing volatile chemical compounds that act as pheromones. Male dogs can detect these signals surprisingly early, sometimes before you notice any physical changes yourself. You may find that male dogs in your neighborhood suddenly become very interested in your yard, or that intact males at the dog park fixate on your dog. Despite all this attention, a dog in proestrus will typically reject mating attempts. She may snap, sit down, or tuck her tail to block males from mounting. This refusal to breed is one of the clearest distinctions between proestrus and the estrus phase that follows.
How Proestrus Differs From Estrus
The key difference is receptivity. During proestrus, your dog attracts males but refuses them. Once estrus begins, she actively accepts mating, often “flagging” her tail to one side when approached. The hormonal shift behind this behavioral change is significant: estrogen drops sharply while progesterone begins to rise, triggering ovulation and the fertile window.
The vaginal discharge also changes. The heavy, bloody discharge of proestrus typically lightens in color and volume as estrus approaches. Some owners mistake this lightening for the end of the heat cycle, but it actually signals the beginning of the fertile period. The combined attractiveness window of proestrus and estrus together lasts roughly 2 to 4 weeks, with an average of about 21 days total.
Signs to Watch For
The most obvious signs of proestrus include:
- Vulvar swelling: The external genitalia become visibly enlarged, sometimes dramatically so.
- Bloody vaginal discharge: This can stain bedding, furniture, and floors. Many owners use dog diapers or keep their dog in easy-to-clean areas during this time.
- Increased urination: Your dog may urinate more frequently, depositing pheromone-rich scent marks that signal her reproductive status to males.
- Behavioral changes: Some dogs become more playful or clingy. Others seem restless or distracted. She may flirt with male dogs but won’t allow mounting.
How Often It Happens
Most dogs go through the complete heat cycle roughly twice a year, though this varies. Smaller breeds may cycle three times per year, while some larger breeds only cycle once every 12 to 18 months. A dog’s first heat typically occurs between 6 and 12 months of age, again with smaller breeds tending to start earlier. The cycle generally becomes more regular after the first year or two but never reaches the clockwork predictability of human menstrual cycles.
When the Cycle Looks Unusual
Not every proestrus follows the textbook pattern. Two variations worth knowing about are silent heats and split heats.
A silent heat occurs when a dog goes through the hormonal changes of proestrus with minimal or no visible signs. There’s little to no discharge and barely any vulvar swelling, so owners may not realize their dog is cycling at all. This is more common in a dog’s first heat cycle.
A split heat is when proestrus begins normally but then stops short. The signs of heat fade, progesterone never rises high enough to trigger ovulation, and the dog appears to go out of heat. Then, one to two months later, the cycle restarts from the beginning. Split heats can be a temporary, harmless quirk, particularly in young dogs still maturing reproductively. In some cases, though, repeated split heats or other irregular patterns can indicate more serious ovarian dysfunction. If your dog’s cycles seem consistently abnormal, that’s worth investigating.
Practical Considerations During Proestrus
Even though your dog isn’t fertile during early proestrus, the transition to estrus isn’t always obvious. The exact day ovulation begins varies from dog to dog, and late proestrus can overlap with early fertility. If you don’t intend to breed your dog, treat the entire heat cycle as a period requiring careful management. Keep her on a leash during walks, avoid off-leash dog parks, and make sure your yard is securely fenced. Male dogs can be remarkably persistent and resourceful when they detect a female in heat.
For breeders trying to time insemination, the shift from proestrus to estrus is critical. Veterinarians can track progesterone levels through simple blood tests to pinpoint ovulation. During proestrus, progesterone stays below 1 ng/mL. When it begins climbing to 2 to 4 ng/mL, that signals the hormonal surge that triggers ovulation, and the fertile window opens shortly after. Getting this timing right matters significantly for breeding success, especially when using shipped or frozen semen.