When withdrawal bleeding and menstruation overlap, it does not lead to a longer period. The reason for withdrawal bleeding is that the medication used contains progestin, such as birth control pills. The main function of progestin is to transform the endometrium from the proliferative phase to the secretory phase, so that the endometrium is in a secretory state and does not increase the thickness of the endometrium, so after stopping the pills, the endometrium will lose the support of progestin and will have withdrawal bleeding. If the use of progestin coincides with the luteal phase of the original menstrual cycle, there will be an overlap between the timing of withdrawal bleeding and menstruation. If the progestin is used during the proliferative phase or during the late luteal phase, there is a risk of prolonged menstrual cycles, increased menstrual flow, and incomplete menstruation.