Prosestrus Stage (6-11 Days)
Vulvar swelling and bleeding (6-11 days)
Your dog will be attractive to males but not receptive to them
This phase is over when your pet becomes receptive to males and will stand to be bred
Estrus Stage (“Fertile” Phase – 5-9 days)
Discharge is straw-colored to light pink
Softer, yet still swollen vulva
This phase is over when your dog is no longer accepting of the male
Diestrus Stage (56-60 days)
A non-receptive time when the corpora lutea, which produce progesterone, are active on the ovary and anestrus.
Anestrus (variable time frame, several months long)
No ovarian activity.
Most dogs cycle about twice a year.
*This information came from the following website: http://www.pets.ca/articles/article-dogpregnant.htm