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The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that makes pregnancy possible. The ovarian cycle controls the production and release of Egg cell and the cyclic release of estrogen and progesterone. The uterine cycle governs the preparation and maintenance of the lining of the uterus (womb) to receive an embryo. These cycles are concurrent and coordinated, normally last between 21 and 35 days, with a median length of 28 days. Menarche (the onset of the first period) usually occurs around the age of 12 years; menstrual cycles continue for about 30–45 years.
Naturally occurring hormones drive the cycles; the cyclical rise and fall of the follicle stimulating hormone prompts the production and growth of (immature egg cells). The hormone estrogen stimulates the uterus lining (endometrium) to thicken to accommodate an embryo should fertilization occur. The blood supply of the thickened lining provides to a successfully implanted embryo. If implantation does not occur, the lining breaks down and blood is released. Triggered by falling progesterone levels, menstruation (commonly referred to as a "period") is the cyclical shedding of the lining, and is a sign that pregnancy has not occurred.
Each cycle occurs in phases based on events either in the ovary (ovarian cycle) or in the uterus (uterine cycle). The ovarian cycle consists of the follicular phase, ovulation, and the luteal phase; the uterine cycle consists of the menstrual, proliferative and secretory phases. Day one of the menstrual cycle is the first day of the period, which lasts for about five days. Around day fourteen, an egg is usually released from the ovary.
The menstrual cycle can cause some women to experience premenstrual syndrome with symptoms that may include tender breasts, and Fatigue. More severe symptoms that affect daily living are classed as premenstrual dysphoric disorder, and are experienced by 3–8% of women. During the first few days of menstruation some women experience Dysmenorrhea that can spread from the abdomen to the back and upper thighs. The menstrual cycle can be modified by hormonal birth control.
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. The cycle is often less regular at the beginning and end of a woman's reproductive life. 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. Menstrual cycles end at menopause, which is usually between 45 and 55 years of age.
After ovulation, the oocyte lives for 24 hours or less without fertilization, 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. Under the influence of progesterone, the uterine lining changes to prepare for potential 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.
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. Although the average length of the human menstrual cycle is similar to that of the lunar cycle, there is Lunar effect between the two.
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. 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).
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.
The release of LH matures the egg and weakens the follicle wall in the ovary, causing the fully developed follicle to release its oocyte. If it is fertilized by a sperm, the oocyte promptly matures into an ootid, which blocks the other spermatozoa and becomes a mature egg. If it is not fertilized by a sperm, the oocyte degenerates. The mature egg has a diameter of about , (2025). 9780815332183, Garland Science. ISBN 9780815332183 and is the largest human cell.
Which of the two ovaries – left or right – ovulates appears random; no left and right coordinating process is known. Occasionally both ovaries release an egg; if both eggs are fertilized, the result is . After release from the ovary into the pelvic cavity, the egg is swept into the fallopian tube by the 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.
Fertilization usually takes place in the ampulla, the widest section of the fallopian tubes. A Zygote immediately starts the process of 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. 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. These hormones also prepare the mammary glands for milk production.
Menstruation is initiated each month by falling levels of estrogen and progesterone and the release of , which constrict the spiral artery. This causes them to spasm, contract and break up. 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. An enzyme called plasmin breaks up the blood clotting in the menstrual fluid, which eases the flow of blood and broken down lining from the uterus. The flow of blood continues for 2–6 days and around 30–60 milliliters of blood is lost, and is a sign that pregnancy has not occurred.
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 bleeding during pregnancy. 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. Menopause is preceded by a stage of hormonal changes called perimenopause.
Eumenorrhea denotes normal, regular menstruation that lasts for around the first 5 days of the cycle. Women who experience menorrhagia (heavy menstrual bleeding) are more susceptible to iron deficiency than the average person.
As estrogen levels increase, cells in the cervix produce a type of cervical mucus that has a higher pH and is less viscosity than usual, rendering it more friendly to sperm. This increases the chances of fertilization, which occurs around day 11 to day 14. This cervical mucus can be detected as a vaginal discharge that is copious and resembles raw egg whites. For women who are practicing fertility awareness, it is a sign that ovulation may be about to take place, but it does not mean ovulation will definitely occur.
If pregnancy does not occur the ovarian and uterine cycles start over again.
There are common culturally communicated misbeliefs that the menstrual cycle affects women's moods, causes depression or irritability, or that menstruation is a painful, shameful or unclean experience. Often a woman's normal mood variation is falsely attributed to the menstrual cycle. Much of the research is weak, but there appears to be a very small increase in mood fluctuations during the luteal and menstrual phases, and a corresponding decrease during the rest of the cycle. Changing levels of estrogen and progesterone across the menstrual cycle exert systemic effects on aspects of physiology including the brain, metabolism, and musculoskeletal system. The result can be subtle physiological and observable changes to women's athletic performance including strength, aerobic, and anaerobic performance.
Changes to the brain have also been observed throughout the menstrual cycle but do not translate into measurable changes in intellectual achievement – including academic performance, problem-solving, and memory. Improvements in Spatial ability during the menstruation phase of the cycle are probably caused by decreases in levels of estrogen and progesterone.
In some women, ovulation features a characteristic pain called mittelschmerz (a German term meaning middle pain). The cause of the pain is associated with the ruptured follicle, causing a small amount of blood loss.
Even when normal, the changes in hormone levels during the menstrual cycle can increase the incidence of disorders such as autoimmune diseases, which might be caused by estrogen enhancement of the immune system.
Around 40% of women with epilepsy find that their occur more frequently at certain phases of their menstrual cycle. This catamenial epilepsy may be due to a drop in progesterone if it occurs during the luteal phase or around menstruation, or a surge in estrogen if it occurs at ovulation. Women who have regular periods can take medication just before and during menstruation. Options include progesterone supplements, increasing the dose of their regular anticonvulsant drug, or temporarily adding an anticonvulsant such as clobazam or acetazolamide. If this is ineffective, or when a woman's menstrual cycle is irregular, then treatment is to stop the menstrual cycle occurring. This may be achieved using medroxyprogesterone, triptorelin or goserelin, or by sustained use of oral contraceptives.
Progestin-only methods of hormonal contraception do not always prevent ovulation but instead work by stopping the cervical mucus from becoming sperm-friendly. Hormonal contraception is available in a variety of forms such as pills, patches, skin implants and Hormonal IUDs (IUDs).
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