No underlying cause is identified in 40-60% of cases of heavy periods (menorrhagia). 

Otherwise, possible causes of heavy periods include the following:

• cervical or endometrial polyps – non-cancerous growths in the lining of the womb or cervix (neck of the womb) 
• endometriosis – when small pieces of the womb lining are found outside the womb, such as in the fallopian tubes, ovaries, bladder or vagina (although this is more likely to cause painful periods)
• uterine fibroids – non-cancerous growths in the womb that can cause pelvic pain
• intrauterine contraceptive devices (IUD) (also known as "the coil") – blood loss may increase by 40-50% after an IUD is inserted
• pelvic inflammatory disease (PID) – an ongoing infection in the pelvis that can cause pelvic pain, fever and bleeding after sexual intercourse or between periods
• polycystic ovary syndrome (PCOS) – women with PCOS typically have a number of cysts in their ovaries
• blood clotting disorders such as Von Willebrand disease
• adenomyosis – when glands from the lining of the uterus become embedded in the uterus muscle
• an underactive thyroid gland (hypothyroidism) – this may cause fatigue, constipation, intolerance to cold, and hair and skin changes
• cancer of the womb (although this is very rare)

Treatments that can cause heavy periods