It is not normal for a 14-year-old girl to have her period last for about 20 days. Generally speaking, the first menstruation of women is roughly between the ages of 11 and 16, and the most common reason for a 14-year-old girl’s menstrual period lasting about 20 days is that the hypothalamus-pituitary-ovarian axis has not yet been established and mature, resulting in pubertal ovulatory disorders of hemorrhage. The most common cause is that the hypothalamus-pituitary-ovarian axis has not yet matured, leading to ovulatory bleeding during puberty. If the patient often stays up too late and is under too much stress, it may lead to endocrine disruption and aggravate the symptoms. If a 14-year-old girl has a period that lasts about 20 days, she should go to the hospital for a checkup to identify the cause of the problem and standardize her treatment.