Editing Menstrual cycle (section)

=== Uterine cycle ===
[[File:Illu cervix.svg|right|thumb|upright=1.6|The anatomy of the uterus]]
The uterine cycle has three phases: menses, proliferative and secretory.<ref name="pmid30521482">{{cite journal |vauthors=Salamonsen LA |title=Women in reproductive science: Understanding human endometrial function |journal=Reproduction |volume=158 |issue=6 |pages=F55–F67 |date=December 2019 |pmid=30521482 |doi=10.1530/REP-18-0518 |doi-access=free |type= Review}}</ref>

==== 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>

==== Proliferative phase ====
[[File:Estradiol and progesterone % changes across the menstrual cycle.tif|thumb|upright=1.6|During the menstrual cycle, levels of estradiol (an estrogen) vary by 200 percent. Levels of progesterone vary by over 1200 percent.<ref>{{cite journal|vauthors=Prior JC|title=Women's reproductive system as balanced estradiol and progesterone actions—A revolutionary, paradigm-shifting concept in women's health|journal=Drug Discovery Today: Disease Models|volume=32 | issue = Part B|year=2020|pages=31–40|doi=10.1016/j.ddmod.2020.11.005|ref=none|doi-access=free}}</ref>]]

The proliferative phase is the second phase of the uterine cycle when estrogen causes the lining of the uterus to grow and proliferate.{{sfn | Ugwumadu | 2014 | p= 117}} The latter part of the follicular phase overlaps with the proliferative phase of the uterine cycle.{{sfn|Parker|2019|p=283}} As they mature, the ovarian follicles secrete increasing amounts of [[estradiol]], an estrogen. The estrogens initiate the formation of a new layer of endometrium in the uterus with the spiral arterioles.{{sfn|Tortora|2017|p=944}}

As estrogen levels increase, cells in the cervix produce a type of [[cervical mucus]]<ref>{{cite journal |vauthors=Simmons RG, Jennings V |title=Fertility awareness-based methods of family planning |journal=Best Practice & Research. Clinical Obstetrics & Gynaecology |volume=66 |pages=68–82 |date=July 2020 |pmid=32169418 |doi=10.1016/j.bpobgyn.2019.12.003 |doi-access=free |type= Review}}</ref> that has a higher [[pH]] and is less [[viscosity|viscous]] than usual, rendering it more friendly to sperm.{{sfn|Tortora|2017|pp=936–937}} This increases the chances of fertilization, which occurs around day 11 to day 14.{{sfn|Tortora|2017|p=957}} This cervical mucus can be detected as a vaginal discharge that is copious and resembles raw egg whites.<ref name= Su2017 /> For women who are practicing [[fertility awareness]], it is a sign that ovulation may be about to take place,<ref name= Su2017>{{cite journal |vauthors=Su HW, Yi YC, Wei TY, Chang TC, Cheng CM |title=Detection of ovulation, a review of currently available methods |journal=Bioeng Transl Med |volume=2 |issue=3 |pages=238–246 |date=September 2017 |pmid=29313033 |pmc=5689497 |doi=10.1002/btm2.10058 |type= Review}}</ref> but it does not mean ovulation will definitely occur.{{sfn|Prior|2020|p=45}}

==== Secretory phase ====
The secretory phase is the final phase of the uterine cycle and it corresponds to the luteal phase of the ovarian cycle. During the secretory phase, the corpus luteum produces progesterone, which plays a vital role in making the endometrium [[Implantation (embryology)#Uterus receptivity|receptive]] to the [[Implantation (embryology)|implantation]] of a [[blastocyst]] (a fertilized egg, which has begun to grow).<ref name="pmid30929718">{{cite journal |vauthors=Lessey BA, Young SL |title=What exactly is endometrial receptivity? |journal=Fertility and Sterility |volume=111 |issue=4 |pages=611–617|date=April 2019 |pmid=30929718 |doi=10.1016/j.fertnstert.2019.02.009 |type= Review|doi-access=free }}</ref> [[Glycogen]], [[lipid]]s, and [[protein]]s are secreted into the uterus<ref name="pmid26661899">{{cite journal |vauthors=Salamonsen LA, Evans J, Nguyen HP, Edgell TA |title=The microenvironment of human implantation: determinant of reproductive success |journal=American Journal of Reproductive Immunology |volume=75 |issue=3 |pages=218–225 |date=March 2016 |pmid=26661899 |doi=10.1111/aji.12450 |type= Review|doi-access=free }}</ref> and the cervical mucus thickens.<ref name="pmid28801053">{{cite journal |vauthors=Han L, Taub R, Jensen JT |title=Cervical mucus and contraception: what we know and what we don't |journal=Contraception |volume=96 |issue=5 |pages=310–321 |date=November 2017 |pmid=28801053 |doi=10.1016/j.contraception.2017.07.168 |type= Review}}</ref> In early pregnancy, progesterone also increases blood flow and reduces the [[Muscle contraction|contractility]] of the [[smooth muscle]] in the uterus{{sfn|Tortora|2017|p= 942}} and raises [[basal body temperature]].<ref name="pmid28488202">{{cite journal |vauthors=Charkoudian N, Hart EC, Barnes JN, Joyner MJ |title=Autonomic control of body temperature and blood pressure: influences of female sex hormones |journal=Clinical Autonomic Research |volume=27 |issue=3 |pages=149–155 |date=June 2017 |pmid=28488202 |doi=10.1007/s10286-017-0420-z |s2cid=3773043 |url=https://research-information.bris.ac.uk/ws/files/152610705/Charkoudian_CAR_review_final.pdf |type=Review |access-date=27 February 2021 |archive-date=10 May 2020 |archive-url=https://web.archive.org/web/20200510013255/https://research-information.bris.ac.uk/ws/files/152610705/Charkoudian_CAR_review_final.pdf |url-status=live |hdl=1983/c0c1058c-553b-4563-8dd1-b047d9b672c1 |hdl-access=free }}</ref>

If pregnancy does not occur the ovarian and uterine cycles start over again.{{sfn|Tortora|2017|page=600}}