There are many reasons for bleeding on the first day of menstruation, which may be related to the presence of foreign bodies in the uterus and abnormal ovarian function. 1. Uterine foreign body: bleeding on the first day of menstruation may be related to the presence of foreign bodies in the uterine cavity, for example, after a recent abortion or curettage, there are residues in the uterus, which may lead to abnormal bleeding phenomenon. 2. Abnormal ovarian function: Bleeding on a clean day may also be caused by abnormal ovarian function. In the late menstrual period, when the estrogen level is low and the endometrium grows slowly, it is easy to bleed continuously, so you can go to the hospital for a sex hormone test and ultrasound to make a judgment. There may also be other reasons for bleeding after the first day of menstruation, so it is recommended to go to the hospital in time, complete the examination to clarify the cause of the disease, and then give targeted treatment or therapy under the guidance of a professional doctor.