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Normal menstruation occurs as the endometrium sloughs from the uterus, with consequent bleeding. The flow lasts for three to five days; in some cases as short as one day, or as long as eight days. Menstruation marks the start of the new menstrual and ovarian cycle.

Menstrual blood is mostly arterial; around a quarter is said to be venous. It also tends to be high in fibrolytic substances; thus menstrual blood tends not to clot. Clots in menstrual blood are associated with particularly heavy bleeding.

Normal blood loss is around 30 mls per cycle; the normal range is up to 80 mls per cycle. This value is chosen since it appears to be the maximum amount that a woman on a normal diet can lose per cycle without becoming iron deficient. In practice, actual blood loss is rarely measured.

Last reviewed 01/2018