Brown blood a few days before menstruation is mainly due to luteal insufficiency. Because after ovulation, the corpus luteum is formed, the progesterone in the body will increase, and the endometrium will thicken under the action of progesterone, and before menstruation, due to the atrophy of the corpus luteum, the progesterone will decline, and the endometrium will have retreating bleeding. When progesterone is insufficient, irregular shedding of the endometrium occurs a few days before menstruation, because the endometrium does not have enough progesterone to support it, so irregular shedding occurs, resulting in brown discharge a few days before menstruation. Clinically, progesterone can be supplemented during the second half of menstruation, which can improve the phenomenon of brown discharge.