After ovulation, during theluteal phase, the egg will be available to befertilized bysperm. If it is not, it will break down in less than a day. Meanwhile, theuterine lining (endometrium) continues to thicken to be able to receive afertilized egg. If noconception occurs, the uterine lining will eventually break down and be shed from the body via thevagina duringmenstruation.[2]
Some people choose to track ovulation in order to improve or aid becoming pregnant by timing intercourse with their ovulation.[3] The signs of ovulation may includecervical mucus changes, mildcramping in the abdominal area, and a small rise inbasal body temperature. Medication is also sometimes required by those experiencing infertility to induce ovulation.[3]
Ovulation occurs about midway through themenstrual cycle, after thefollicular phase. The days in which a woman is most fertile can be calculated based on the date of the last menstrual period and the length of a typical menstrual cycle.[4] The few days surrounding ovulation (from approximately days 10 to 18 of a 28-day cycle), constitute the most fertile phase.[5][6][7][8] The time from the beginning of thelast menstrual period (LMP) until ovulation is, on average, 14.6 days, but with substantial variation among females and between cycles in any single female, with an overall 95%prediction interval of 8.2 to 20.5 days.[9]
The process of ovulation is controlled by thehypothalamus of the brain and through the release of hormones secreted in theanterior lobe of thepituitary gland,luteinizing hormone (LH) andfollicle-stimulating hormone (FSH).[10] In thepreovulatory phase of themenstrual cycle, the ovarian follicle will undergo a series of transformations called cumulus expansion, which is stimulated by FSH. After this is done, a hole called thestigma will form in thefollicle, and the secondary oocyte will leave the follicle through this hole. Ovulation is triggered by a spike in the amount of FSH and LH released from the pituitary gland. During theluteal (post-ovulatory) phase, the secondary oocyte will travel through thefallopian tubes toward theuterus. Iffertilized by asperm, the fertilized secondary oocyte or ovum mayimplant there 6–12 days later.[11]
Thefollicular phase (or proliferative phase) is the phase of the menstrual cycle during which theovarian follicles mature. The follicular phase lasts from the beginning ofmenstruation to the start of ovulation.[12][13]
For ovulation to be successful, the ovum must be supported by thecorona radiata andcumulus oophorousgranulosa cells.[14] The latter undergo a period of proliferation and mucification known as cumulus expansion. Mucification is the secretion of ahyaluronic acid-rich cocktail that disperses and gathers the cumulus cell network in a sticky matrix around the ovum. This network stays with the ovum after ovulation and has been shown to be necessary for fertilization.[15][16]
Estrogen levels peak towards the end of the follicular phase, around 12 and 24 hours. This, by positive feedback, causes a surge in levels ofluteinizing hormone (LH) andfollicle-stimulating hormone (FSH). This lasts from 24 to 36 hours, and results in the rupture of the ovarian follicles, causing the oocyte to be released from the ovary.[17]
Through a signal transduction cascade initiated by LH, which activates the pro-inflammatory genes through cAMP secondary messenger,proteolytic enzymes are secreted by the follicle that degrade the follicular tissue at the site of the blister, forming a hole called thestigma. Thesecondary oocyte leaves the ruptured follicle and moves out into theperitoneal cavity through the stigma, where it is caught by thefimbriae at the end of thefallopian tube. After entering the fallopian tube, the oocyte is pushed along bycilia, beginning its journey toward theuterus.[10]
By this time, the oocyte has completedmeiosis I, yielding two cells: the largersecondary oocyte that contains all of the cytoplasmic material and a smaller, inactive first polar body.Meiosis II follows at once but will be arrested in themetaphase and will so remain until fertilization. Thespindle apparatus of the second meiotic division appears at the time of ovulation. If no fertilization occurs, the oocyte will degenerate between 12 and 24 hours after ovulation.[18] Approximately 1–2% of ovulations release more than one oocyte. This tendency increases with maternal age. Fertilization of two different oocytes by two different spermatozoa results in fraternal twins.[10]
The precise moment of ovulation was captured on film for the first time in 2008, coincidentally, during a routine hysterectomy procedure. According to the attending gynecologist, the ovum's emergence and subsequent release from the ovarian follicle occurred within a 15-minute timeframe.[19]
The follicle proper has met the end of its lifespan. Without the oocyte, the follicle folds inward on itself, transforming into thecorpus luteum (pl. corpora lutea), a steroidogenic cluster of cells that producesestrogen andprogesterone. These hormones induce the endometrial glands to begin production of theproliferative endometrium and later intosecretory endometrium, the site of embryonic growth if implantation occurs. The action of progesterone increasesbasal body temperature by one-quarter to one-half degree Celsius (one-half to one degree Fahrenheit). The corpus luteum continues thisparacrine action for the remainder of the menstrual cycle, maintaining the endometrium, before disintegrating into scar tissue during menses.[20]
The start of ovulation may be detected by signs that are not readily discernible other than to the ovulating female herself, thus humans are said to have aconcealed ovulation.[21] In many animal species there are distinctive signals indicating the period when the female is fertile. Several explanations have been proposed to explain concealed ovulation in humans.
Females near ovulation experience changes in thecervical mucus, and inbasal body temperature. Furthermore, many females experience secondary fertility signs includingMittelschmerz (pain associated with ovulation) and a heightened sense ofsmell, and can sense the precise moment of ovulation.[22][23] However, midcycle pain may also not be due to Mittelschmerz, but due to other factors such as cysts, endometriosis, sexually transmitted infections, or an ectopic pregnancy.[24] Other possible signs of ovulation include tender breasts, bloating, and cramps, although these symptoms are not a guarantee that ovulation is taking place.[25][26]
Many females experience heightened sexual desire in the several days immediately before ovulation.[27] One study concluded that females subtly improve their facial attractiveness during ovulation.[28]
Symptoms related to the onset of ovulation, the moment of ovulation and the body's process of beginning and ending the menstrual cycle vary in intensity with each female but are fundamentally the same. The charting of such symptoms — primarily basal body temperature, mittelschmerz and cervical position — is referred to as the sympto-thermal method of fertility awareness, which allow auto-diagnosis by a female of her state of ovulation. Once training has been given by a suitable authority, fertility charts can be completed on a cycle-by-cycle basis to show ovulation. This gives the possibility of using the data to predict fertility for natural contraception and pregnancy planning.
Urine levels of thehormone pregnanediol 3-glucuronide of over 5 μg/mL has been used to confirm ovulation. This test has a 100% specificity over 107 women.[30]
Disorders of ovulation, also known asovulatory disorders are classified asmenstrual disorders and include oligoovulation (infrequent or irregular ovulation) and anovulation (absence of ovulation):[31]
Oligoovulation is infrequent or irregular ovulation (usually defined as cycles of greater than 36 days or fewer than 8 cycles a year)
Anovulation is absence of ovulation when it would be normally expected (in a post-menarchal, premenopausal female). Anovulation usually manifests itself as irregularity of menstrual periods, that is, unpredictable variability of intervals, duration, or bleeding. Anovulation can also cause cessation of periods (secondary amenorrhea) or excessive bleeding (dysfunctional uterine bleeding).
WHO group II: Hypothalamic–pituitary–gonadal axis dysfunction. WHO group II is the most common cause of ovulatory disorders, and the most common causative member ispolycystic ovary syndrome (PCOS).[33]
A low dose ofhuman chorionic gonadotropin (HCG) may be injected after completed ovarian stimulation. Ovulation will occur between 24 and 36 hours after the HCG injection.[35]
By contrast,induced ovulation in some animal species occurs naturally, ovulation can be stimulated by coitus.[36]
Most women who are able to conceive are fertile for an estimated five days before ovulation and one day after ovulation.[39] There is some evidence that for couples who have been trying to conceive a child for less than 12 months, and the female is under 40 years old, practicing timed intercourse (timing intercourse with ovulation using urine tests that predict ovulation) may help improve the rate of pregnancy and live births.[39] The role that stress plays in ovulation, fertility, and understanding the biological basis for stress-induced anovulation and the role of cortisol is not entirely clear.[40]
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^Bullivant SB, Sellergren SA, Stern K, Spencer NA, Jacob S, Mennella JA, McClintock MK (February 2004). "Women's sexual experience during the menstrual cycle: identification of the sexual phase by noninvasive measurement of luteinizing hormone".Journal of Sex Research.41 (1):82–93.doi:10.1080/00224490409552216.PMID15216427.S2CID40401379.
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^Baird, D. T.; Balen, A.; Escobar-Morreale, H. F.; Evers, J. L. H.; Fauser, B. C. J. M.; Franks, S.; Glasier, A.; Homburg, R.; La Vecchia, C.; Devroey, P.; Diedrich, K.; Fraser, L.; Gianaroli, L.; Liebaers, I.; Sunde, A.; Tapanainen, J. S.; Tarlatzis, B.; Van Steirteghem, A.; Veiga, A.; Crosignani, P. G.; Evers, J. L. H. (2012). "Health and fertility in World Health Organization group 2 anovulatory women".Human Reproduction Update.18 (5):586–99.doi:10.1093/humupd/dms019.PMID22611175.
^Nelson AL, Cwiak C (2011). "Combined oral contraceptives (COCs)". In Hatcher RA, Trussell J, Nelson AL, Cates W, Kowal D, Policar MS (eds.).Contraceptive technology (20th revised ed.). New York: Ardent Media. pp. 249–341.ISBN978-1-59708-004-0.ISSN0091-9721.OCLC781956734. pp. 257–258
^Endrikat J, Gerlinger C, Richard S, Rosenbaum P, Düsterberg B (December 2011). "Ovulation inhibition doses of progestins: a systematic review of the available literature and of marketed preparations worldwide".Contraception.84 (6):549–57.doi:10.1016/j.contraception.2011.04.009.PMID22078182.
Chabbert Buffet N, Djakoure C, Maitre SC, Bouchard P (July 1998). "Regulation of the human menstrual cycle".Frontiers in Neuroendocrinology.19 (3):151–86.doi:10.1006/frne.1998.0167.PMID9665835.S2CID40594356.
