The correct medical definition of heavy periods is the passage of more than 80ml of blood each period. It is seldom realistic or practical, however, to actually measure the blood loss and so doctors rely on the woman's description of her period.

Periods are considered heavy when:
- the period lasts for more than 8 to 10 days, especially if this is repeated month after month.
- it affects everyday activities (e.g. work, holidays, hobbies).
- the bleeding is continuously so heavy that the woman becomes anaemic.
- there are clots and flooding.

Common causes for heavy periods include hormonal reasons, endometriosis, pelvic inflammatory disease and in particular, uterine fibroids.