Irregular menstruation in a 16-year-old girl may be caused by puberty physiological reaction, polycystic ovary syndrome, excessive stress and other reasons. 1. Physiological reaction of puberty: the central feedback mechanism of estrogen is imperfect in pubertal girls, and the ovaries do not ovulate, causing irregular menstruation. 2. Polycystic ovary syndrome: polycystic ovary syndrome is most common in puberty, the onset of which may be related to genetic factors, environmental factors and so on. Polycystic ovary syndrome is characterized by high androgen levels, ovarian failure to ovulate and other causes of irregular menstruation, obesity, acanthosis nigricans and so on. 3. Excessive stress: prolonged stress, tension, overwork, emotional changes and other factors lead to neuroendocrine disorders in the girl’s body, causing irregular menstruation in girls. 16-year-old girl menstrual irregularities may also be other reasons, it is recommended to consult a regular hospital, under the guidance of the doctor to identify the causes, give appropriate treatment.