Menorrhagia has been classically defined as blood loss of ≥80 mL per month during cyclic menses.  As menstrual blood volume is difficult to gauge, the term is commonly used to denote excessive volume and/or excessive duration (>1 week) of menstrual bleeding. The condition is common among women of reproductive age. Prevalence increases with age, peaking between 30 and 49 years of age, when about 1 in 20 women each year consults a general practitioner for bleeding.    
In the UK, 20% of women have a hysterectomy by the age of 60, mainly for heavy bleeding, despite the fact that 40% have a normal uterus on histological examination.  In the US, 20% to 25% of healthy pre-menopausal women have abnormal uterine bleeding.  Rates of menorrhagia in non-Western countries are unknown. Abnormal uterine bleeding accounts for about 25% of gynaecological surgeries. 
During normal cyclic menstrual bleeding, oestrogen and progesterone from the ovary induce the production of prostaglandins, cytokines, and matrix metalloproteinases (MMPs). These are directly responsible for the cyclic regeneration of the functional layer of the endometrium.  Abnormal uterine bleeding represents a disruption in this orderly progression.  Thinning of the vascular smooth muscle cell layer of the spiral arterioles, shifts in prostaglandin secretion towards vasodilatory prostaglandins, and disturbances in the endometrial coagulation mechanisms are often found in women with heavy menstrual bleeding. Menorrhagia may occur in ovulatory cycles that are typically regular. Heavy bleeding associated with irregular cycles is more likely to represent anovulatory bleeding, caused by a host of distinct conditions. Menorrhagia may occur without any identifiable structural, hormonal, haematological, or other systemic abnormality.