* Proestrus: This is the first stage of the heat cycle and it usually lasts for 3 to 9 days. During this stage, the dog's vulva will begin to swell and she may experience some vaginal discharge. She will also start to attract male dogs.
* Estrus: This is the second stage of the heat cycle and it usually lasts for 9 to 12 days. During this stage, the dog is fertile and can become pregnant. She will continue to experience vaginal discharge and her vulva will be swollen. She will also be very attractive to male dogs and may try to mount them.
* Diestrus: This is the third stage of the heat cycle and it usually lasts for 12 to 14 days. During this stage, the dog's body is preparing for a possible pregnancy. She may experience some vaginal discharge and her vulva will be swollen. She will also be less attractive to male dogs.
* Anestrus: This is the fourth and final stage of the heat cycle and it usually lasts for 4 to 6 months. During this stage, the dog's body is resting and preparing for the next heat cycle. She will not experience any vaginal discharge and her vulva will be small and closed.
The length of a small dog's heat cycle can vary depending on the breed of dog and her individual health. If you have any questions about your dog's heat cycle, you should talk to your veterinarian.