Irregular menstruation in 15-year-old girls may be caused by physiological factors, mental factors, malnutrition and other reasons. 1. Physiological factors: 15-year-old girls have just started menstruating, because the development of the pituitary gonadal axis is not yet fully mature, so irregular menstruation is usually a normal phenomenon, with the development of the pituitary gonadal axis will gradually improve. 2. Mental factors: 15 years old is in the age of learning, mental overstress, learning pressure is relatively large, may also lead to irregular menstruation. 3. Malnutrition: Some girls are picky eaters, partiality, lack of nutrition, etc., can also lead to irregular menstruation. 15-year-old girl irregular menstruation may also exist other reasons, if the girl comes to menstruation irregular for a long time, it is recommended to timely consult a doctor to do the relevant examination, under the guidance of professional doctors to clarify the cause of the disease, targeted treatment.