Normally,the process of blood forming clots occurs(coagulation) to limit and repair the damaged blood vessel resulting in a stoppage of blood flow. Menorrhagia is an abnormally heavy and prolonged menstrual period caused by abnormal blood clotting, disruption of normal hormonal regulation of periods, or disorders of the endometrial lining of the uterus. 
Since normal menstrual cycle about 28 days in duration, with bleeding lasting an average of total blood loss less than 80 mL, women with menorrhagia over the menstruation may have blood loss greater than the body's iron reserves and reproduced by the body leading to anemia