Editing Menstrual cycle (section)

==== Menstruation ====
{{main|Menstruation}}
Menstruation (also called menstrual bleeding, menses or a period) is the first and most evident phase of the uterine cycle and first occurs at puberty. Called menarche, the first period occurs at the age of around twelve or thirteen years.<ref name="pmid26703478">{{cite journal |vauthors=Papadimitriou A |title=The evolution of the age at menarche from prehistorical to modern times |journal=Journal of Pediatric and Adolescent Gynecology |volume=29 |issue=6 |pages=527–530 |date=December 2016 |pmid=26703478 |doi=10.1016/j.jpag.2015.12.002 |type= Review}}</ref> The average age is generally later in the [[developing world]] and earlier in the [[developed world]].<ref name="pmid29778270">{{cite journal |vauthors=Alvergne A, Högqvist Tabor V |title=Is female health cyclical? Evolutionary perspectives on menstruation |journal=Trends in Ecology & Evolution |volume=33 |issue=6 |pages=399–414 |date=June 2018 |pmid=29778270 |doi=10.1016/j.tree.2018.03.006 |arxiv=1704.08590 |bibcode=2018TEcoE..33..399A |s2cid=4581833 |type= Review}}</ref>  In [[precocious puberty]], it can occur as early as age eight years,<ref name="pmid28591132">{{cite journal |vauthors=Ibitoye M, Choi C, Tai H, Lee G, Sommer M |title=Early menarche: A systematic review of its effect on sexual and reproductive health in low- and middle-income countries |journal=PLOS ONE |volume=12 |issue=6 |pages=e0178884 |date=2017 |pmid=28591132 |pmc=5462398 |doi=10.1371/journal.pone.0178884 |bibcode=2017PLoSO..1278884I |type= Review|doi-access=free }}</ref> and this can still be normal.<ref name=Women2014Men>{{cite web|title=Menstruation and the menstrual cycle fact sheet|url=http://www.womenshealth.gov/publications/our-publications/fact-sheet/menstruation.html|website=Office of Women's Health |publisher= US Department of Health and Human Services |access-date=25 June 2015|date=23 December 2014|url-status=dead|archive-url=https://web.archive.org/web/20150626134338/http://www.womenshealth.gov/publications/our-publications/fact-sheet/menstruation.html|archive-date=26 June 2015}}</ref><ref name="pmid29422239">{{cite journal |vauthors=Sultan C, Gaspari L, Maimoun L, Kalfa N, Paris F |title=Disorders of puberty |journal=Best Practice & Research. Clinical Obstetrics & Gynaecology |volume=48 |issue= |pages=62–89 |date=April 2018 |pmid=29422239 |doi=10.1016/j.bpobgyn.2017.11.004 |url=https://hal.umontpellier.fr/hal-01797379/file/2018%20Sultan%20et%20al.%2C%20Disorders%20of%20puberty.pdf |type=Review |access-date=27 February 2021 |archive-date=1 July 2020 |archive-url=https://web.archive.org/web/20200701081342/https://hal.umontpellier.fr/hal-01797379/file/2018%20Sultan%20et%20al.,%20Disorders%20of%20puberty.pdf |url-status=live }}</ref>

Menstruation is initiated each month by falling levels of estrogen and progesterone and the release of [[prostaglandin]]s,{{sfn|Tortora|2017|p=945}} which constrict the [[spiral artery|spiral arteries]]. This causes them to [[spasm]], contract and break up.{{sfn|Johnson|2007|p=152}} The blood supply to the endometrium is cut off and the cells of the top layer of the endometrium (the stratum functionalis) become deprived of oxygen and die. Later the whole layer is lost and only the bottom layer, the stratum basalis, is left in place.{{sfn|Tortora|2017|page=945}} An [[enzyme]] called [[plasmin]] breaks up the [[blood clotting|blood clots]] in the menstrual fluid, which eases the flow of blood and broken down lining from the uterus.{{sfn | Tortora | 2017 | p=600}} The flow of blood continues for 2–6 days and around 30–60 [[milliliters]] of blood is lost,{{sfn|Prior|2020|p=45}} and is a sign that pregnancy has not occurred.{{sfn|Johnson|2007|p=99}}

The flow of blood normally serves as a sign that a woman has not become pregnant, but this cannot be taken as certainty, as several factors can cause [[Vaginal bleeding#Pregnant women|bleeding during pregnancy]].<ref name="pmid27166462">{{cite journal |vauthors=Breeze C |title=Early pregnancy bleeding |journal=Australian Family Physician |volume=45 |issue=5 |pages=283–286 |date=May 2016 |pmid=27166462 |type= Review}}</ref> Menstruation occurs on average once a month from menarche to menopause, which corresponds with a woman's fertile years. The average age of menopause in women is 52 years, and it typically occurs between 45 and 55 years of age.<ref name="pmid27022074">{{cite journal |vauthors=Towner MC, Nenko I, Walton SE |title=Why do women stop reproducing before menopause? A life-history approach to age at last birth |journal=Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences |volume=371 |issue=1692 |page=20150147 |date=April 2016 |pmid=27022074 |pmc=4822427 |doi=10.1098/rstb.2015.0147 |type= Review}}</ref> Menopause is preceded by a stage of hormonal changes called [[perimenopause]].{{sfn | Rodriguez-Landa | 2017 | p=8}}

''Eumenorrhea'' denotes normal, regular menstruation that lasts for around the first 5 days of the cycle.{{sfn|Tortora|2017|p=943}} Women who experience [[menorrhagia]] (heavy menstrual bleeding) are more susceptible to [[iron deficiency (medicine)|iron deficiency]] than the average person.<ref name=iron>{{cite journal | vauthors = Harvey LJ, Armah CN, Dainty JR, Foxall RJ, John Lewis D, Langford NJ, Fairweather-Tait SJ | title = Impact of menstrual blood loss and diet on iron deficiency among women in the UK | journal = The British Journal of Nutrition | volume = 94 | issue = 4 | pages = 557–564 | date = October 2005 | pmid = 16197581 | doi = 10.1079/BJN20051493 | doi-access = free| type= Comparative study}}</ref>