Menstruation
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.
Reference
- Munro MG, Critchley HOD, Fraser IS., FIGO Menstrual Disorders Committee. The two FIGO systems for normal and abnormal uterine bleeding symptoms and classification of causes of abnormal uterine bleeding in the reproductive years: 2018 revisions. Int J Gynaecol Obstet. 2018 Dec;143(3):393-408.
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