Menstruation in less than a month may be a normal physiological phenomenon, or it may be caused by endocrine disorders, submucosal fibroids and so on. 1. Normal physiological phenomenon: the normal menstrual cycle is 21 to 35 days, if the patient is within this range, then it is normal. If the patient’s menstruation is early in this month only occasionally, it may be caused by not having a good rest, too much stress in the mind or taking drugs such as progesterone, and usually it will return to normal in the next time of menstruation. 2. Pathological phenomena: (1) Endocrine disorders: the most common is the premature atrophy of the corpus luteum caused by insufficient progesterone, when the endometrium does not have enough progesterone to support it, resulting in the phenomenon of partial detachment, which can lead to menstruation in less than a month. (2) Submucosal fibroids: when fibroids grow into the uterine cavity, the endometrium will increase in size, resulting in shortening of menstrual cycle and the phenomenon of menstruation coming in less than a month. It is recommended that when the patient has menstruation in less than a month, normal physiological phenomena should be excluded and the patient should go to the hospital for medical consultation to clarify the cause of the disease, and treatment should be carried out under the guidance of the doctor.