Unlike human females, whose menstrual cycles typically take 28 days, dogs enter their heat cycles, or estrus, on average only twice a year. For dogs, the complete estrus cycle typically lasts between two and three weeks. Three main stages make up your dog’s heat cycle, each with different symptoms.
When your dog’s estrus cycle begins, the external vulva begins to swell. In many dogs, this swelling is not noticeable; many owners are unaware their dog has gone into estrus.
The second part of estrus is often the first one owners notice. This is when the bloody vaginal discharge begins. Discharge amounts vary by breed and dog; they can be heavy or hardly noticeable. This bloody discharge typically lasts between seven and 10 days.
After the bleeding stops, vaginal discharge continues but changes to a watery pink substance. This is the time when the female dog ovulates and is most fertile. You may notice frequent urination as she marks her scent to attract male dogs.