Editing Menstrual cycle (section)

== Cycles and phases ==
[[File:Figure 28 02 07.jpg|thumb|upright=1.5|right|Progression of the menstrual cycle and some of the hormones contributing to it]]

The menstrual cycle encompasses the ovarian and uterine cycles. The ovarian cycle describes changes that occur in the [[Ovarian follicle|follicles]] of the [[ovary]],<ref name="pmid29544627">{{cite journal|vauthors=Richards JS|date=2018|title=The ovarian cycle|journal=Vitamins and Hormones|volume=107|issue=|pages=1–25|doi=10.1016/bs.vh.2018.01.009|isbn=978-0-128-14359-9|pmid=29544627 |type= Review}}</ref> whereas the uterine cycle describes changes in the [[Endometrium|endometrial lining]] of the uterus.  Both cycles can be divided into phases.  The ovarian cycle consists of alternating [[follicular phase|follicular]] and [[luteal phase]]s, and the uterine cycle consists of the [[menstruation|menstrual phase]], the proliferative phase, and the secretory phase.{{sfn|Tortora|2017|p=944}} The menstrual cycle is controlled by the [[hypothalamus]] in the brain, and the [[anterior pituitary gland]] at the base of the brain. The hypothalamus releases [[gonadotropin-releasing hormone]] (GnRH), which causes the nearby anterior pituitary to release [[follicle-stimulating hormone]] (FSH) and [[luteinizing hormone]] (LH). Before [[puberty]], GnRH is released in low steady quantities and at a steady rate. After puberty, GnRH is released in large pulses, and the frequency and magnitude of these determine how much FSH and LH are produced by the pituitary.{{sfn|Prior|2020|p=42}}

Measured from the first day of one menstruation to the first day of the next, the length of a menstrual cycle varies but has a [[median]] length of 28 days.<ref name="Reed2018" /> The cycle is often less regular at the beginning and end of a woman's reproductive life.<ref name="Reed2018" /> At puberty, a child's body begins to mature into an adult body capable of [[sexual reproduction]]; the first period (called [[menarche]]) occurs at around 12 years of age and continues for about 30–45 years.{{sfn|Prior|2020|p=40}}<ref name="pmid29261991">{{cite journal |vauthors=Lacroix AE, Gondal H, Langaker MD |title=Physiology, menarche |journal=StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing |year=2020 |pmid=29261991|type= Review }}</ref> Menstrual cycles end at [[menopause]], which is usually between 45 and 55 years of age.{{sfn | Rodriguez-Landa | 2017 | p=8}}<ref name="pmid26703478" />

=== Ovarian cycle ===
Between menarche and menopause the ovaries regularly alternate between luteal and follicular phases during the monthly menstrual cycle.{{sfn | Sherwood | 2016 | p=741}} Stimulated by gradually increasing amounts of [[estrogen]] in the follicular phase, discharges of blood flow stop and the uterine lining thickens. Follicles in the ovary begin developing under the influence of a complex interplay of hormones, and after several days one, or occasionally two, become dominant, while non-dominant follicles shrink and die. About mid-cycle, some 10–12 hours after the increase in luteinizing hormone, known as the LH surge,<ref name= Reed2018>{{cite journal |vauthors= Reed BF, Carr BR, Feingold KR, et al |title= The Normal Menstrual Cycle and the Control of Ovulation |journal= Endotext |date= 2018 |pmid= 25905282 |url= https://www.ncbi.nlm.nih.gov/books/NBK279054/?report=classic |type= Review |access-date= 8 January 2021 |archive-date= 28 May 2021 |archive-url=https://web.archive.org/web/20210528022539/https://www.ncbi.nlm.nih.gov/books/NBK279054/?report=classic |url-status= live }}</ref> the dominant follicle releases an [[oocyte]], in an event called [[ovulation]].{{sfn | Sherwood | 2016 | p=747}}

After ovulation, the oocyte lives for 24 hours or less without [[Human fertilization|fertilization]],{{sfn|Tortora|2017|p=957}} while the remains of the dominant follicle in the ovary become a [[corpus luteum]] –  a body with the primary function of producing large amounts of the hormone [[progesterone]].{{sfn|Tortora|2017|p=929}}{{efn|Progesterone levels exceed those of estrogen (estradiol) by a hundred-fold.{{sfn|Prior|2020|p=41}}}} Under the influence of progesterone, the uterine lining changes to prepare for potential [[Implantation (embryology)|implantation]] of an [[embryo]] to establish a pregnancy. The thickness of the endometrium continues to increase in response to mounting levels of estrogen, which is released by the [[antral follicle]] (a mature ovarian follicle) into the blood circulation. Peak levels of estrogen are reached at around day thirteen of the cycle and coincide with ovulation. If implantation does not occur within about two weeks, the corpus luteum degenerates into the [[corpus albicans]], which does not produce hormones, causing a sharp drop in levels of both progesterone and estrogen. This drop causes the uterus to lose its lining in menstruation; it is around this time that the lowest levels of estrogen are reached.{{sfn|Tortora|2017|pp=942–946}}

In an ovulatory menstrual cycle, the ovarian and uterine cycles are concurrent and coordinated and last between 21 and 35 days, with a population average of 27–29 days.{{sfn|Prior|2020|p=45}} Although the average length of the human menstrual cycle is similar to that of the [[lunar cycle]], there is [[Lunar effect|no causal relation]] between the two.{{sfn|Norris|Carr|2013|p= [https://books.google.com/books?id=F_NaW1ZcSSAC&pg=PA361 361]}}

==== Follicular phase ====
{{main|Follicular phase}}
The ovaries contain a finite number of [[Oogonium|egg stem cells]], [[granulosa cells]] and [[Theca of follicle|theca cells]], which together form primordial follicles.{{sfn|Tortora|2017|p=929}} At around 20 weeks into [[gestation]] some 7 million immature eggs have already formed in an ovary. This decreases to around 2&nbsp;million by the time a girl is born, and 300,000 by the time she has her first period. On average, one egg matures and is released during ovulation each month after menarche.{{sfn | Ugwumadu | 2014 | p=115}} Beginning at puberty, these mature to primary follicles independently of the menstrual cycle.{{sfn | Watchman | 2020 | p=8}}  The development of the egg is called [[oogenesis]] and only one cell survives the [[Prophase#Meiotic prophase|divisions]] to await fertilization. The other cells are discarded as [[polar body|polar bodies]], which cannot be fertilized.<ref name="pmid21268179">{{cite journal |vauthors=Schmerler S, Wessel GM |title=Polar bodies – more a lack of understanding than a lack of respect |journal=Molecular Reproduction and Development |volume=78 |issue=1 |pages=3–8 |date=January 2011 |pmid=21268179 |pmc=3164815 |doi=10.1002/mrd.21266 |type= Review}}</ref> The follicular phase is the first part of the ovarian cycle and it ends with the completion of the [[antral follicles]].{{sfn | Sherwood | 2016 | p=741}} [[Meiosis]] (cell division) remains incomplete in the egg cells until the antral follicle is formed. During this phase usually only one ovarian follicle fully matures and gets ready to release an egg.{{sfn | Tortora | 2017 | p=945}} The follicular phase shortens significantly with age, lasting around 14 days in women aged 18–24 compared with 10 days in women aged 40–44.{{sfn | Tortora | 2017 | pp=942–946}}

Through the influence of a rise in [[follicle stimulating hormone]] (FSH) during the first days of the cycle, a few ovarian follicles are stimulated. These follicles, which have been developing for the better part of a year in a process known as [[folliculogenesis]], compete with each other for dominance. All but one of these follicles will stop growing, while one dominant follicle – the one that has the most FSH receptors –  will continue to maturity. The remaining follicles die in a process called [[follicular atresia]].{{sfn|Johnson|2007|page=86}} [[Luteinizing hormone]] (LH) stimulates further development of the ovarian follicle. The follicle that reaches maturity is called an antral follicle, and it contains the [[ovum]] (egg cell).{{sfn|Tortora|2017|p=942}}

The theca cells develop receptors that bind LH, and in response secrete large amounts of [[androstenedione]]. At the same time the granulosa cells surrounding the maturing follicle develop receptors that bind FSH, and in response start secreting androstenedione, which is converted to estrogen by the enzyme [[aromatase]]. The estrogen inhibits further production of FSH and LH by the pituitary gland. This [[negative feedback]] regulates levels of FSH and LH. The dominant follicle continues to secrete estrogen, and the rising estrogen levels make the pituitary more responsive to GnRH from the hypothalamus. As estrogen increases this becomes a [[positive feedback]] signal, which makes the pituitary secrete more FSH and LH. This surge of FSH and LH usually occurs one to two days before ovulation and is responsible for stimulating the rupture of the antral follicle and release of the oocyte.{{sfn | Watchman | 2020 | p=8}}{{sfn | Sherwood | 2016 | p=745}}

==== Ovulation ====
{{main|Ovulation}}
[[File:ovulation.jpg|thumb|An ovary about to release an egg]]
Around day fourteen, the egg is released from the ovary.{{sfn | Tortora | 2017 | p=943}}  Called [[ovulation]], this occurs when a mature egg is released from the ovarian follicles into the pelvic cavity and enters the [[fallopian tube]], about 10–12 hours after the peak in LH surge.<ref name= Reed2018/> Typically only one of the 15–20 stimulated follicles reaches full maturity, and just one egg is released.{{sfn | Sadler | 2019 | p=48}} Ovulation only occurs in around 10% of cycles during the first two years following menarche, and by the age of 40–50, the number of ovarian follicles is depleted.{{sfn|Tortora|2017|p=953}} LH initiates ovulation at around day 14 and stimulates the formation of the corpus luteum.{{sfn|Tortora|2017|p=944}} Following further stimulation by LH, the corpus luteum produces and releases estrogen, progesterone, [[relaxin]] (which relaxes the uterus by inhibiting contractions of the [[myometrium]]), and [[inhibin]] (which inhibits further secretion of FSH).{{sfn|Tortora|2017|p=920}}

The release of LH matures the egg and weakens the follicle wall in the ovary, causing the fully developed follicle to release its oocyte.{{sfn | Sherwood | 2016 | p=746}} If it is fertilized by a sperm, the oocyte promptly matures into an [[ootid]], which blocks the other [[spermatozoa|sperm cells]] and becomes a mature egg. If it is not fertilized by a sperm, the oocyte degenerates. The mature egg has a diameter of about {{cvt|0.1|mm}},<ref>{{Cite book| vauthors = Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P |date=2002|chapter=Eggs|title=Molecular Biology of the Cell|edition=4th|isbn=0-8153-3218-1|chapter-url= https://www.ncbi.nlm.nih.gov/books/NBK26842/ |location=New York|publisher=Garland Science |access-date=25 February 2021 |archive-date=16 December 2019 |archive-url= https://web.archive.org/web/20191216042524/https://www.ncbi.nlm.nih.gov/books/NBK26842/ |url-status=live}}</ref> and is the largest human cell.<ref name="pmid30739329">{{cite journal | vauthors = Iussig B, Maggiulli R, Fabozzi G, Bertelle S, Vaiarelli A, Cimadomo D, Ubaldi FM, Rienzi L | title = A brief history of oocyte cryopreservation: Arguments and facts | journal = Acta Obstetricia et Gynecologica Scandinavica | volume = 98 | issue = 5 | pages = 550–558 | date = May 2019 | pmid = 30739329 | doi = 10.1111/aogs.13569 | type = Review | doi-access = free }}</ref>

Which of the two ovaries – left or right – ovulates appears random;{{sfn | Parker | 2019 | p=283}} no left and right coordinating process is known.{{sfn|Johnson|2007|pp=192–193}} Occasionally both ovaries release an egg; if both eggs are fertilized, the result is [[fraternal twin]]s.{{sfn|Johnson|2007|p=192}} After release from the ovary into the pelvic cavity, the egg is swept into the fallopian tube by the [[Fimbria (female reproductive system)|fimbria]] –  a fringe of tissue at the end of each fallopian tube. After about a day, an unfertilized egg disintegrates or dissolves in the fallopian tube, and a fertilized egg reaches the uterus in three to five days.{{sfn | Sadler | 2019 | p=36}}

Fertilization usually takes place in the [[Ampulla of uterine tube|ampulla]], the widest section of the fallopian tubes. A [[Zygote|fertilized egg]] immediately starts the process of [[Human embryonic development|embryonic development]]. The developing embryo takes about three days to reach the uterus, and another three days to implant into the endometrium. It has reached the [[blastocyst]] stage at the time of implantation: this is when pregnancy begins.{{sfn | Tortora | 2017 | p=959}} The loss of the corpus luteum is prevented by fertilization of the egg.  The [[syncytiotrophoblast]] (the outer layer of the resulting embryo-containing blastocyst that later becomes the outer layer of the placenta) produces [[human chorionic gonadotropin]] (hCG), which is very similar to LH and preserves the corpus luteum. During the first few months of pregnancy, the corpus luteum continues to secrete progesterone and estrogens at slightly higher levels than those at ovulation. After this and for the rest of the pregnancy, the [[placenta]] secretes high levels of these hormones – along with hCG, which stimulates the corpus luteum to secrete more progesterone and estrogens, blocking the menstrual cycle.{{sfn|Tortora|2017|page=976}} These hormones also prepare the mammary glands for milk{{efn|[[Breastfeeding]] women can experience complete suppression of follicular development, follicular development but no ovulation, or resumption of normal menstrual cycles.<ref name="pmid25869631">{{cite journal |vauthors=Carr SL, Gaffield ME, Dragoman MV, Phillips S |title=Safety of the progesterone-releasing vaginal ring (PVR) among lactating women: A systematic review |journal=Contraception |volume=94 |issue=3 |pages=253–261 |date=September 2016 |pmid=25869631 |doi=10.1016/j.contraception.2015.04.001 |doi-access=free |type= Review}}</ref>}} production.{{sfn|Tortora|2017|page=976}}

==== Luteal phase ====
{{main|Luteal phase}}
Lasting about 14 days,<ref name="Reed2018" /> the luteal phase is the final phase of the ovarian cycle and it corresponds to the secretory phase of the uterine cycle. During the luteal phase, the pituitary hormones FSH and LH cause the remaining parts of the dominant follicle to transform into the corpus luteum, which produces progesterone.{{sfn|Johnson|2007|page=91}}{{efn|In the corpus luteum, [[cholesterol side-chain cleavage enzyme]] converts [[cholesterol]] to [[pregnenolone]], which is converted to progesterone.<ref name="pmid22201776">{{cite journal |vauthors=King SR, LaVoie HA |title=Gonadal transactivation of STARD1, CYP11A1 and HSD3B |journal=Frontiers in Bioscience (Landmark Edition) |volume=17 |issue= 3|pages=824–846 |date=January 2012 |pmid=22201776 |doi=10.2741/3959 |doi-access=free }}</ref>}} The increased progesterone starts to induce the production of estrogen. The hormones produced by the corpus luteum also suppress production of the FSH and LH that the corpus luteum needs to maintain itself. The level of FSH and LH fall quickly, and the corpus luteum atrophies.{{sfn | Ugwumadu | 2014 | p=117}} Falling levels of progesterone trigger menstruation and the beginning of the next cycle. For an individual woman, the follicular phase often varies in length from cycle to cycle; by contrast, the length of her luteal phase will be fairly consistent from cycle to cycle at 10 to 16 days (average 14 days).{{sfn | Tortora | 2017 | pp=942–946}}

=== 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}}