A nevus is medically known as a pigmented mole. Causes of pigmented nevus on forehead include UV factors, genetic factors, endocrine factors, and tumors. Pigmented nevus is a kind of benign tumor composed of nevus cells, which can be seen in any part of the body. 1. Ultraviolet ray factor: If a person is exposed to the sun for a long time, the ultraviolet ray will stimulate the skin and lead to the proliferation of skin tissues so that the pigmented nevus will grow. 2. Hereditary factors: if parents have a nevus on their forehead, the probability of their children having a pigmented nevus on their forehead will also be higher. 3. endocrine factors: due to often stay up late, irregular life and routine lead to endocrine disorders, hormone stimulation of the skin triggered by skin tissue proliferation, will also appear the phenomenon of long pigmented nevus. 4. Tumor: pigmented nevus may also be the manifestation of early skin tumor. If the pigmented nevus lesions on the skin have rapid growth, change in pigmentation, irregular shape, satellite nevus around, etc., it suggests that the nature of the nevus is malignant or malignant lesions may occur. You should consult a doctor for treatment in time. There may be other reasons for the appearance of a pigmented nevus on the forehead, such as changes in the pigmented nevus lesions, you should consult a doctor in time.