Proestrus is the initial stage of the reproductive cycle in female mammals, including dogs and cats. During this phase, which typically lasts about 9 days, the body prepares for potential mating. Hormonal changes lead to the thickening of the uterine lining and the development of follicles in the ovaries. In dogs, signs of proestrus include swelling of the vulva and a bloody discharge. This stage is crucial as it sets the stage for estrus, the period when the female is receptive to mating. Understanding proestrus helps pet owners recognize their animals' reproductive cycles.