Some patients will have their periods twice a month. If you have your period twice a month, the most common cause is ovulatory bleeding. Normally, menstruation usually occurs once every 28 days or so. The underlying cause of menstruation is the shedding of the endometrium. If there is partial shedding of the endometrium in the middle of two periods, it is often due to the loss of hormonal support of the endometrium due to a brief drop in estrogen levels during ovulation. There is no need to worry and no special treatment is needed.