Editing Menstruation (section)

===Length and duration===
The first menstrual period occurs after the onset of pubertal growth, and is called [[menarche]]. The average age of menarche is 12 to 15 years.<ref name="Jones2011"/><ref>{{cite journal | vauthors = Karapanou O, Papadimitriou A | title = Determinants of menarche | journal = Reproductive Biology and Endocrinology | volume = 8 | pages = 115 | date = September 2010 | pmid = 20920296 | pmc = 2958977 | doi = 10.1186/1477-7827-8-115 | doi-access = free }}</ref> However, it may occur as early as eight.<ref name="Women2014Men"/> The average age of the first period is generally later in the [[developing world]], and earlier in the [[developed world]].<ref name=Diaz2006/><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 }}</ref> The average age of menarche has changed little in the United States since the 1950s.<ref name=Diaz2006/>

Menstruation is the most visible phase of the [[menstrual cycle]] and its beginning is used as the marker between cycles. The first day of menstrual bleeding is the date used for the last menstrual period (LMP). The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women, and 21 to 35 days in adults.<ref name="Women2014Men"/><ref name="Diaz2006" /> The average length is 28 days; one study estimated it at 29.3 days.<ref name=":6">{{cite journal | vauthors = Bull JR, Rowland SP, Scherwitzl EB, Scherwitzl R, Danielsson KG, Harper J | title = Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles | journal = npj Digital Medicine | volume = 2 | issue = 1 | pages = 83 | date = 2019-08-27 | pmid = 31482137 | pmc = 6710244 | doi = 10.1038/s41746-019-0152-7 }}</ref> The variability of menstrual cycle lengths is highest for women under 25 years of age and is lowest, that is, most regular, for ages 25 to 39 years.<ref name="Chiazze1968">{{cite journal | vauthors = Chiazze L, Brayer FT, Macisco JJ, Parker MP, Duffy BJ | title = The length and variability of the human menstrual cycle | journal = JAMA | volume = 203 | issue = 6 | pages = 377–380 | date = February 1968 | pmid = 5694118 | doi = 10.1001/jama.1968.03140060001001 }}</ref> The variability increases slightly for women aged 40 to 44 years.<ref name="Chiazze1968" />

[[Perimenopause]] is when a woman's fertility declines, and menstruation occurs less regularly in the years leading up to the final menstrual period, when a woman stops menstruating completely and is no longer fertile. The medical definition of [[menopause]] is one year without a period and typically occurs between 45 and 55 years in Western countries.<ref name="NIH2013Def" /><ref name = "Carlson_2004">{{cite book |vauthors= Carlson KJ, Eisenstat SA, Ziporyn TD |title = The new Harvard guide to women's health | year = 2004 | publisher = Harvard University Press | location = Cambridge, Massachusetts | isbn = 0-674-01343-3 | url-access = registration | url = https://archive.org/details/newharvardguidet00carl }}</ref>{{rp|381}} Menopause before age 45 is considered ''premature'' in industrialized countries.<ref>{{cite web|title=Clinical topic&nbsp;— Menopause|url=http://www.cks.nhs.uk/menopause#-292420|url-status=dead|archive-url=https://web.archive.org/web/20090707095123/http://www.cks.nhs.uk/menopause#-292420|archive-date=7 July 2009|access-date=2 November 2009|work=NHS}}</ref> Illnesses, certain surgeries, or medical treatments may cause menopause to occur earlier than it might have otherwise.<ref name="pmid31027683">{{cite journal | vauthors = Mishra GD, Chung HF, Cano A, Chedraui P, Goulis DG, Lopes P, Mueck A, Rees M, Senturk LM, Simoncini T, Stevenson JC, Stute P, Tuomikoski P, Lambrinoudaki I | display-authors = 6 | title = EMAS position statement: Predictors of premature and early natural menopause | journal = Maturitas | volume = 123 | issue =  | pages = 82–88 | date = May 2019 | pmid = 31027683 | doi = 10.1016/j.maturitas.2019.03.008 | hdl-access = free | s2cid = 87361924 | hdl = 10138/318039 }}</ref>