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:
- a woman bleeds for more than 8 to 10 days, especially if this is repeated month after month.
- a woman bleeds so much that it is difficult for her to attend her job. She may be forced - to plan her holidays and leisure time according to the timings of her period.
- the bleeding is continuously so heavy that the woman becomes anaemic.
- the presence of other than small clots for more than one or two days suggests heavy periods.
- 'flooding' describes the sudden, unexpected onset of periods, like turning on a tap, and indicates heavy periods.

Common causes include:

Hormonal imbalance. In a normal menstrual cycle, a balance between the hormones estrogen and progesterone regulates the buildup of the lining of the uterus (endometrium), which is shed during menstruation. If a hormonal imbalance occurs, the endometrium develops in excess and eventually sheds by way of heavy menstrual bleeding.

Dysfunction of the ovaries. If ovulation does not occur in a menstrual cycle (anovulation), progesterone is not produced. This causes hormonal imbalance and may result in menorrhagia.
Uterine fibroids. These noncancerous (benign) tumors of the uterus appear during your childbearing years. Uterine fibroids may cause heavier than normal or prolonged menstrual bleeding.

Polyps. Small, benign growths on the lining of the uterine wall (uterine polyps) may cause heavy or prolonged menstrual bleeding. Polyps of the uterus most commonly occur in women of reproductive age as the result of high hormone levels.

Endometrial disorders. The disorders including endometriosis, adenomyosis, and endometrial hyperplasia. Any of them occurs when glands from the endometrium become embedded in the uterine muscle, often causing heavy bleeding and/or painful menses. Adenomyosis is most likely to develop if you're a middle-aged woman who has had children, while endometrial hyperplasia can occur on even teenagers.  

Intrauterine device (IUD). Menorrhagia is a well-known side effect of using a nonhormonal intrauterine device for birth control. When an IUD is the cause of excessive menstrual bleeding, you may need to remove it.

Pregnancy complications. A single, heavy, late period may be due to a miscarriage. If bleeding occurs at the usual time of menstruation, however, miscarriage is unlikely to be the cause. An ectopic pregnancy — implantation of a fertilized egg within the fallopian tube instead of the uterus — also may cause menorrhagia.

Cancer. Rarely, uterine cancer, ovarian cancer and cervical cancer can cause excessive menstrual bleeding.

Inherited bleeding disorders. Some blood coagulation disorders — such as von Willebrand's disease, a condition in which an important blood-clotting factor is deficient or impaired — can cause abnormal menstrual bleeding.

Medications. Certain drugs, including anti-inflammatory medications and anticoagulants, can contribute to heavy or prolonged menstrual bleeding. Improper use of hormone medications also can cause menorrhagia.

Other medical conditions. A number of other medical conditions, including pelvic inflammatory disease (PID), thyroid problems, endometriosis, and liver or kidney disease, may be associated with menorrhagia.