The cause of brown blood in the underpants a week before menstruation may be pregnancy, inflammation, luteal insufficiency and so on. 1. Pregnancy: If you are a woman of childbearing age and have sex, you can consider pregnancy, due to abnormal levels of estrogen and progesterone in the body after pregnancy, or early miscarriage, there may be symptoms of brown blood in the underpants one week before menstruation, and you should consult a doctor in time for an early pregnancy test. 2. Inflammation: such as vaginal inflammation, cervical inflammation, endometrial inflammation, etc., may also cause brown blood in the underwear one week before menstruation, which can be examined after the end of menstruation. 3. Luteal insufficiency: The week before menstruation is mainly the luteal phase, which may be due to the insufficient function of the corpus luteum, resulting in insufficient secretion of progesterone, thus leading to abnormal bleeding, manifested as brown blood in the underwear a week before menstruation. If there is brown blood in the underwear a week before menstruation, you can seek medical attention in time.