Possible reasons for not having menstrual periods include pregnancy, irregular menstruation during puberty, taking medications and other reasons. 1. Pregnancy: Healthy women in their reproductive years with a history of sexual intercourse who normally have regular menstrual cycles should be considered to be pregnant if they do not have their periods. If the menstrual period does not come for more than 10 days, pregnancy should be highly suspected and can be determined by urine pregnancy test or blood HCG (human chorionic gonadotropin). 2. Irregular menstruation during puberty: Irregular menstruation after menarche is mainly due to the fact that the gonadal system is not fully developed during puberty, and is affected by factors such as environment, emotion, and stress, which leads to irregular menstrual cycles, and menstruation will only become normal gradually after regular cyclic ovulation is established. 3. Taking medication: Taking birth control pills can affect the menstrual cycle, leading to delayed menstruation. 4. Other: Possible causes include genital tuberculosis, polycystic ovary syndrome, prolonged strenuous exercise, menopause and so on. There are many reasons why menstruation does not come, found that menstruation can be observed for a week, if it does not come, it is recommended to consult a doctor in a timely manner, to clarify the cause of timely treatment under the guidance of the doctor.