There are many reasons why menstruation does not flow out, as follows: 1, pregnancy status: women of childbearing age, if they have sex after clean menstruation and do not do contraception, to exclude the case of unwanted pregnancy, generally more than 12 days after intercourse can use a pregnancy test or early pregnancy test paper to detect. 2, endocrine disorders: if pregnancy is completely excluded, it may also be caused by endocrine disorders. If women often stay up late during their periods, mental stress, nervousness, bad mood or change of environment, these abnormalities may also affect ovulation, resulting in menstruation not arriving as scheduled.3. After abortion: If women do not have normal menstruation for more than 35 days after having an abortion, it may be caused by residual uterine cavity or another unplanned pregnancy. Of course, if it is thin endometrium, cervical adhesions or cervical canal adhesions, it may also lead to failure of menstruation.4. Menopause: If a woman is between 45-55 years old and cannot have menstruation for more than 12 months, she can be diagnosed as menopause.