A nevus is also known as a “pigmented mole”. Reasons for the sudden increase in the size of a nevus include ultraviolet radiation, trauma, repeated external stimulation, and malignant changes. 1. Ultraviolet radiation: long-term exposure of pigmented nevus to strong ultraviolet rays will easily stimulate melanin synthesis, which in turn will cause pigmented nevus to expand. 2. Trauma: If the local skin where the nevus of pigmentation is located is scratched, abraded and other traumatic stimuli, and the wound breaks down and bleeds, the nevus of pigmentation may also become bigger after healing. 3. Repeated external stimulation: if the nevus is in a part that is easy to be rubbed repeatedly, or if the patient scratches it repeatedly, the area of the nevus can also be increased. 4. Malignant transformation: If the nevus becomes bigger and grows rapidly, it can be considered as malignant transformation. Patients need to undergo relevant examinations in time to find out whether the nevus has undergone malignant changes. It is recommended that the patient should consult the doctor and undergo relevant examinations in time, and receive treatment according to the doctor’s instruction after the cause of the disease is clarified.