Brown vaginal discharge with fishy odor may be precancerous, and its common causes include inflammation of the reproductive system, reproductive system tumors, and genitourinary fistula. 1. Inflammation of the reproductive system: Inflammation of the reproductive system is mostly due to the invasion of pathogenic microorganisms into the vagina, destroying the balance of vaginal flora. When anaerobic bacteria in the vagina continue to metabolize and produce amines, you can spread the fishy smell. It is common in vaginitis or cervicitis caused by various pathogenic microorganisms such as Trichomonas vaginalis, Pseudohyphae, Gonorrhea and so on. 2. Tumor of reproductive system: a large amount of necrosis of tumor tissue can produce fishy smell, which is mostly seen in malignant tumors of reproductive system such as vaginal cancer, cervical cancer, endometrial cancer and so on. 3. Genitourinary fistula: fistula formed between the urinary tract and reproductive tract, which can lead to urine or other excreta partially excreted through the vagina, and also produce vaginal odor, commonly seen in vesicovaginal fistula, urethrovaginal fistula, rectovaginal fistula and so on.