The average cycle for a female dog in heat happens about once every six months, typically between January and March and again between August and October, beginning when a dog reaches about 6 months old. There are four different stages for the estrus (heat) cycle.
Proestrus (First Stage)
Your dog bleeds during the proestrus stage of its cycle, which typically lasts about seven to 10 days. You might notice that your dog urinates more and its vulva is swollen. If your dog has a heavy bleeding flow, you can visit your local pet store and purchase dog panties that have special sanitary napkins in them.
Estrus (Second Stage)
The estrus stage, which is when your dog is willing to breed, typically lasts five to nine days. You may notice that when you touch your dog's back, it stands still and moves its tail to the side. If you don't want your dog to breed, you need to keep close tabs on it during this stage to ensure it doesn't seek out or attract a mate.
Diestrus (Third Stage)
During the diestrus stage, which typically lasts six to 10 weeks, your dog shows few outward signs of the cycle. Everything that happens during this stage is all internal; a lot of ovarian activity occurs. One outward sign that your dog may show is a false pregnancy.
Anestrus (Final Stage)
This is the period between your dog's heat cycle. It lasts about 15 weeks, during which there is no hormonal activity and the dog is not willing to mate.