Editing Menstrual cycle (section)

==Evolution and other species==
{{further|Menstruation (mammal)}}
Most [[female mammals]] have an [[estrous cycle]], but only ten primate species, four [[bat]] species, the [[elephant shrew]]s and the [[Cairo spiny mouse]] (''Acomys cahirinus'') have a [[Menstruation (mammal)|menstrual cycle]].<ref name = Bellofiore>{{cite journal | vauthors = Bellofiore N, Ellery SJ, Mamrot J, Walker DW, Temple-Smith P, Dickinson H | title = First evidence of a menstruating rodent: the spiny mouse&nbsp;(Acomys&nbsp;cahirinus) | journal = American Journal of Obstetrics and Gynecology | volume = 216 | issue = 1 | pages = 40.e1–40.e11 | date = January 2017 | pmid = 27503621 | doi = 10.1016/j.ajog.2016.07.041 | s2cid = 88779 |type= Journal article| url = https://www.biorxiv.org/content/biorxiv/early/2016/06/03/056895.full.pdf }}</ref><ref>{{Cite journal|last1=Bellofiore|first1=Nadia|last2=Cousins|first2=Fiona|last3=Temple-Smith|first3=Peter|last4=Evans|first4=Jemma|date=2019-02-01|title=Altered exploratory behaviour and increased food intake in the spiny mouse before menstruation: a unique pre-clinical model for examining premenstrual syndrome|url=https://academic.oup.com/humrep/article/34/2/308/5248532|journal=Human Reproduction|language=en|volume=34|issue=2|pages=308–322|doi=10.1093/humrep/dey360|pmid=30561655|issn=0268-1161|doi-access=free}}</ref> The cycles are the same as in humans apart from the length, which ranges from 9 to 37 days.<ref name="pmid32129461"/><ref name = Bellofiore/> The lack of immediate relationship between these groups suggests that four distinct evolutionary events have caused menstruation to arise.<ref name=Emera2012/> There are four theories on the evolutionary significance of menstruation:<ref name=Emera2012>{{cite journal |vauthors=Emera D, Romero R, Wagner G |title=The evolution of menstruation: a new model for genetic assimilation: explaining molecular origins of maternal responses to fetal invasiveness |journal=BioEssays |volume=34 |issue=1 |pages=26–35 |date=January 2012 |pmid=22057551 |pmc=3528014 |doi=10.1002/bies.201100099 |type=Journal article}} See [https://web.archive.org/web/20210801180159/http://www.bbc.com/earth/story/20150420-why-do-women-have-periods BBC Earth lay summary], 20 April 2015.</ref>

# Control of sperm-borne pathogens.<ref name="pmid18046752">{{cite journal |vauthors=Martin RD |title=The evolution of human reproduction: a primatological perspective |journal=American Journal of Physical Anthropology |volume=134 |issue=S45 |pages=59–84 |date=2007 |pmid=18046752 |doi=10.1002/ajpa.20734 |s2cid=44416632 |type= Review|doi-access=free }}</ref><ref name="pmid9618925">{{cite journal |vauthors=Finn CA |title=Menstruation: a nonadaptive consequence of uterine evolution |journal=The Quarterly Review of Biology |volume=73 |issue=2 |pages=163–173 |date=June 1998|pmid=9618925|doi=10.1086/420183 |s2cid=25135630 |type= Review}}</ref><ref name="pmid8210311">{{cite journal |vauthors=Profet M |title=Menstruation as a defense against pathogens transported by sperm |journal=The Quarterly Review of Biology |volume=68 |issue=3 |pages=335–386 |date=September 1993 |pmid=8210311 |doi=10.1086/418170 |s2cid=23738569 |type= Review}}</ref>  This hypothesis held that menstruation protected the uterus against [[pathogen]]s introduced by [[sperm]]. Hypothesis 1 does not take into account that [[Sexual intercourse|copulation]] can take place weeks before menstruation and that potentially infectious [[semen]] is not controlled by menstruation in other species.<ref name=Emera2012/>
# Energy conservation.<ref name="pmid9618925"/><ref name="pmid8693059">{{cite journal |vauthors=Strassmann BI |title=The evolution of endometrial cycles and menstruation |journal=The Quarterly Review of Biology |volume=71 |issue=2 |pages=181–220 |date=June 1996 |pmid=8693059 |doi=10.1086/419369 |s2cid=6207295 |type= Review}}</ref> This hypothesis claimed that it took less energy to rebuild a uterine lining than to maintain it if pregnancy did not occur.  Hypothesis&nbsp;2 does not explain other species that also do not maintain a uterine lining but do not menstruate.<ref name= Emera2012/>
# A theory based on spontaneous decidualization (a process that results in significant changes to cells of the endometrium in preparation for, and during, pregnancy). Decidualization leads to the differentiation of the endometrial stroma, which involves cells of the immune system,<ref name="pmid32129461">{{cite journal |vauthors=Catalini L, Fedder J |title=Characteristics of the endometrium in menstruating species: lessons learned from the animal kingdom† |journal=Biology of Reproduction |volume=102 |issue=6 |pages=1160–1169 |date=May 2020 |pmid=32129461 |pmc=7253787 |doi=10.1093/biolre/ioaa029 |type= Journal article}}</ref> the formation of a new blood supply, hormones and tissue differentiation. In non-menstruating mammals, decidualization is driven by the embryo, not the mother.<ref>{{Cite journal |last1=Muter |first1=Joanne |last2=Kong |first2=Chow-Seng |last3=Brosens |first3=Jan J. |date=2021-12-23 |title=The Role of Decidual Subpopulations in Implantation, Menstruation and Miscarriage |journal=Frontiers in Reproductive Health |language=English |volume=3 |doi=10.3389/frph.2021.804921 |doi-access=free |pmid=36303960 |pmc=9580781 |issn=2673-3153}}</ref> According to this theory, menstruation is an unintended consequence of the decidualization process and the body uses spontaneous decidualization to identify and reject defective embryos early on.<ref name=":0">{{Cite journal |last1=Teklenburg |first1=Gijs |last2=Salker |first2=Madhuri |last3=Molokhia |first3=Mariam |last4=Lavery |first4=Stuart |last5=Trew |first5=Geoffrey |last6=Aojanepong |first6=Tepchongchit |last7=Mardon |first7=Helen J. |last8=Lokugamage |first8=Amali U. |last9=Rai |first9=Raj |last10=Landles |first10=Christian |last11=Roelen |first11=Bernard A. J. |last12=Quenby |first12=Siobhan |last13=Kuijk |first13=Ewart W. |last14=Kavelaars |first14=Annemieke |last15=Heijnen |first15=Cobi J. |date=2010-04-21 |title=Natural Selection of Human Embryos: Decidualizing Endometrial Stromal Cells Serve as Sensors of Embryo Quality upon Implantation |journal=PLOS ONE |volume=5 |issue=4 |pages=e10258 |doi=10.1371/journal.pone.0010258 |doi-access=free |issn=1932-6203 |pmc=2858159 |pmid=20422011|bibcode=2010PLoSO...510258T }}</ref> This process happens because the decidual cells of the stroma can recognize and respond to defects in a developing embryo by stopping the secretion of [[cytokine]]s needed for the embryo to implant.<ref name=":0" />
# Uterine pre-conditioning.<ref name="pmid19136085">{{cite journal |vauthors=Brosens JJ, Parker MG, McIndoe A, Pijnenborg R, Brosens IA |title=A role for menstruation in preconditioning the uterus for successful pregnancy |journal=American Journal of Obstetrics and Gynecology |volume=200 |issue=6 |pages=615.e1–6 |date=June 2009 |pmid=19136085 |doi=10.1016/j.ajog.2008.11.037 |type= Journal article}}</ref> This hypothesis claims that a monthly pre-conditioning of the uterus is needed in species, such as humans, that have deeply invasive (deep-rooted) [[placenta]]s. In the process leading to the formation of a placenta, maternal tissues are invaded. This hypothesis holds that menstruation was not evolutionary, rather the result of a coincidental pre-conditioning of the uterus to protect uterine tissue from the deeply rooting placenta, in which a thicker endometrium develops.<ref name="pmid19136085"/> Hypothesis&nbsp;4 does not explain menstruation in non-primates.<ref name=Emera2012/>