Menstruation is considered normal when uterine bleeding occurs every 21 to 35 days and is not excessive. The normal duration of menstrual bleeding is between two and seven days. Abnormal uterine bleeding occurs when either the frequency or quantity of uterine bleeding differs from that mentioned above or the woman has spotting in between her menstrual periods. Abnormal uterine bleeding is caused by a variety of factors. The two main causes are structural abnormalities of the reproductive system and ovulation disorders.