In unspayed dogs, the first estrus cycle, or heat, usually occurs between 6 and 24 months. In smaller breeds, the first estrus is often earlier than in larger breeds.

A female dog's behavior changes markedly during estrus. She becomes more affectionate and may appear nervous or agitated. More frequent urination is also an indicator. A male dog is typically attracted to a female in estrus for approximately 18 days, but she is attracted for only half of this duration. When the female dog is fertile, her vaginal discharge changes from light red to straw-colored and decreases in volume.

To prevent estrus, a puppy can be spayed as early as 2 months.