There are several reasons why menstruation occurs twice: First, consider the endometrium, vagina problems, normal menstruation can not be separated from the vagina, uterus and ovaries of the normal cooperation, if any one of the organs have problems, it will lead to problems with the amount of menstruation, the duration of menstruation, there will be a double menstruation. Second, consider the endocrine system, normal menstruation cannot be separated from the precise regulation of the endocrine system, during menstruation, estrogen, luteinizing hormone will have a large change in the body, if the amount of hormone changes, the regulation of menstruation will be impaired, so that the duration of menstruation does not occur or come to the menstrual period twice, or even many times menstruation.