Menstruation refers to menstruation.Occasional irregularity of menstruation in a 20-year-old girl may be caused by physiological factors, endocrine disorders, polycystic ovary syndrome and other factors. 1. Physiological factors: the average cycle of women’s menstruation is about 28 days, but there may be delayed or early, usually within 21-35 days, as long as it does not exceed the value of this range, it is generally normal. 2. Endocrine disorders: 20-year-old girls with irregular diets, excessive psychological stress, and frequent late nights may cause endocrine disorders, leading to disruptions in the menstrual cycle, which can make menstruation occasionally irregular, and may improve on their own after eliminating the above triggers. 3. Polycystic ovary syndrome: it may be related to genetic factors, metabolic factors, environmental factors, once polycystic ovary syndrome occurs, it will have an impact on women’s menstrual cycle, resulting in irregular menstruation. If a 20-year-old girl has occasional irregular menstruation, and if it is accompanied by other uncomfortable symptoms at the same time, it is recommended to consult a doctor for examination and treatment.