Editing Menstrual cycle (section)

==== 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}}