Understanding the Heat Cycle in More Detail A female dog's heat cycle  is part of her reproductive process and consists of different phases, each with its own hormonal and physical changes. 📌 The Four Stages of the Canine Heat Cycle 1️⃣ Proestrus (Preparation Phase)  – Lasts 7–10 days The vulva begins swelling , and light bleeding starts. The dog attracts males , but she is not yet ready  for mating. Hormones, particularly estrogen , are rising, preparing the body for ovulat