When your intact female dog completes her heat cycle, you'll notice a change in her vulva's appearance. When she's about to enter the cycle, you'll see a different change. When she's about to enter heat, or estrus, her vulva enlarges. When she's not in heat, her vulva returns to normal size. The vulva, a female mammal's external genitalia, contains her vagina.
The initial, 10-day phase of the canine estrus cycle consists of proestrus, when her vulva swells and there's a bloody discharge. Actual estrus lasts just a few days, but during this time the dog ovulates and experiences peak fertility. Diestrus, the third phase, lasts six to 10 weeks. The vulva starts returning to normal size. If she's not pregnant, the dog enters the anestrus stage, lasting about four months. She has no interest in breeding and her vulva remains the same size.
Jane Meggitt has been a writer for more than 20 years. In addition to reporting for a major newspaper chain, she has been published in "Horse News," "Suburban Classic," "Hoof Beats," "Equine Journal" and other publications. She has a Bachelor of Arts in English from New York University and an Associate of Arts from the American Academy of Dramatics Arts, New York City.