|Classification and external resources|
|ICD-10||N92.1, N92.5, N92.6|
Irregular cycles or periods 
Irregular cycles or irregular periods is an abnormal variation in length of menstrual cycles in a woman. A woman usually experiences cycle length variations of up to eight days between the shortest and longest cycle lengths. Length variation between eight and 20 days is considered as moderately irregular cycles. Variation of 21 days or more is considered very irregular.
Alternatively, a single menstruation period may be defined as irregular if it is shorter than 21 days, or longer than 36 days. However, if they are regularly shorter than 21 days, or longer than 36 (or 35) days, the condition would rather be termed polymenorrhea or oligomenorrhea, respectively.
Other types 
Other types of conditions that can be referred to by "irregular menstruation" include:
- Metrorrhagia, which generally refers to vaginal bleeding that occurs between the expected menstrual periods. The distinction between irregular cycle lengths and metrorrhagia is not always clear, and may depend on whether the bleeding is regarded as marking the menstrual period (favoring the term "irregular cycles") or being separate from it (favoring the term "metrorrhagia").
- Oligomenorrhea generally refers to infrequent menstruation, More strictly, it is menstrual periods occurring at intervals of greater than 35 days, with only four to nine periods in a year. Also, menstrual periods should have been regularly established previously before the development of infrequent flow. and often (but not always) involves irregular intervals. In contrast to "irregular cycles", the interval between one cycle and the next may still be consistent, but can still be regarded as "irregular" compared to the cycle length of a woman without oligomenorrhea. Still, women with oligomenorrhea often have irregular cycles as well.
- Polymenorrhea is the medical term for cycles with intervals of 21 days or fewer. This can be regarded as the opposite of oligomenorrhea.
- Kippley, John; Sheila Kippley (1996). The Art of Natural Family Planning (4th ed.). Cincinnati, OH: The Couple to Couple League. p. 92. ISBN 0-926412-13-2.
- about.com > If I Have Irregular Periods, Will I Have Trouble Getting Pregnant? By Rachel Gurevich. Updated October 26, 2009
- thefreedictionary.com > oligomenorrhea This dictionary, in turn, is citing: Gale Encyclopedia of Medicine. 2008
- MedicineNet.com > Definition of Metrorrhagia Last Editorial Review: 3/17/2003