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84-439: In demographic contexts, fertility refers to the actual production of offspring, rather than the physical capability to produce which is termed fecundity . While fertility can be measured, fecundity cannot be. Demographers measure the fertility rate in a variety of ways, which can be broadly broken into "period" measures and " cohort " measures. "Period" measures refer to a cross-section of the population in one year. "Cohort" data on

168-499: 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 oocytes (immature egg cells). The hormone estrogen stimulates the uterus lining ( endometrium ) to thicken to accommodate an embryo should fertilization occur. The blood supply of

252-502: A patriotic duty to help increase their country's birthrate. Even the government was reluctant in its support to the movement. It was only between 1938 and 1939 that the French government became directly and permanently involved in the pronatalist effort. Although the birthrate started to surge in late 1941, the trend was not sustained. Falling birthrate once again became a major concern among demographers and government officials beginning in

336-410: 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

420-432: A larger clutch size to compensate for the reduced reproduction frequency, thus increasing those species' fecundity. Fecundity is a significant component of fitness . Fecundity selection builds on that idea. This idea claims that the genetic selection of traits that increase an organism's fecundity is, in turn, advantageous to an organism's fitness. Fecundity Schedule Fecundity Schedules are data tables that display

504-515: A lesser degree) increased male age . The "Three-step Analysis" of the fertility process was introduced by Kingsley Davis and Judith Blake in 1956 and makes use of three proximate determinants: The economic analysis of fertility is part of household economics , a field that has grown out of the New Home Economics . Influential economic analyses of fertility include Becker (1960), Mincer (1963), and Easterlin (1969). The latter developed

588-485: 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

672-471: A part of its reproductive strategy. These species produce many offspring during their one reproductive event, giving them a potential advantage when it comes to fecundity, as they are producing more offspring. Iteroparity is when a species reproduces multiple times over its lifetime. This species' strategy is to protect against the unpredictable survivability of their offspring, in which if their first litter of offspring dies, they can reproduce again and replace

756-400: 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. The average age is generally later in the developing world and earlier in the developed world . In precocious puberty , it can occur as early as age eight years, and this can still be normal. Menstruation

840-462: 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 no causal relation between the two. The ovaries contain a finite number of egg stem cells , granulosa cells and theca cells , which together form primordial follicles. At around 20 weeks into gestation some 7 million immature eggs have already formed in an ovary. This decreases to around 2 million by

924-496: 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

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1008-455: A short luteal phase (less than ten days ) in which progesterone production is insufficient for normal physiology and fertility. Cycles in which ovulation does not occur ( anovulation ) are common in girls who have just begun menstruating and in women around menopause. During the first two years following menarche, ovulation is absent in around half of cycles. Five years after menarche, ovulation occurs in around 75% of cycles and this reaches 80% in

1092-1121: A year, while a study in 2013 of 2,820 Danish women saw 78% of 35- to 40-year-olds conceive within a year. According to an opinion by the Practice Committee of the American Society for Reproductive Medicine, specific coital timing or position, and resting supine after intercourse have no significant impact on fertility. Sperm can be found in the cervical canal seconds after ejaculation, regardless of coital position. Successful pregnancies facilitated by fertility treatment have been documented in women as old as 67 . Some research suggests that older males have decreased semen volume, sperm motility , and impaired sperm morphology. In studies that controlled for female partner's age, comparisons between men under 30 and men over 50 found relative decreases in pregnancy rates between 23% and 38%. Sperm count declines with age, with men aged 50–80 years producing sperm at an average rate of 75% compared with men aged 20–50 years and larger differences exist in

1176-504: Is a sign that ovulation may be about to take place, but it does not mean ovulation will definitely occur. 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 receptive to the implantation of a blastocyst (a fertilized egg, which has begun to grow). Glycogen , lipids , and proteins are secreted into

1260-631: Is available in a variety of forms such as pills, patches , skin implants and hormonal intrauterine devices (IUDs). Most female mammals have an estrous cycle , but only ten primate species, four bat species, the elephant shrews and the Cairo spiny mouse ( Acomys cahirinus ) have a menstrual cycle . The cycles are the same as in humans apart from the length, which ranges from 9 to 37 days. The lack of immediate relationship between these groups suggests that four distinct evolutionary events have caused menstruation to arise. There are four theories on

1344-442: Is considered impossible to cease reproduction based on social factors, and fecundity tends to rise after a brief decline. Fecundity has also been shown to increase in ungulates with relation to warmer weather. In sexual evolutionary biology , especially in sexual selection, fecundity is contrasted to reproductivity . Fertile window The menstrual cycle is a series of natural changes in hormone production and

1428-414: 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

1512-434: Is felt as painful cramps in the abdomen that can spread to the back and upper thighs during the first few days of menstruation. Debilitating period pain is not normal and can be a sign of something severe such as endometriosis . These issues can significantly affect a woman's health and quality of life and timely interventions can improve the lives of these women. There are common culturally communicated misbeliefs that

1596-429: Is generally a consistent feature of each culture. Fecundation is another term for fertilization . In obstetrics and gynecology , fecund-ability is the probability of being pregnant in a single menstrual cycle , and fecundity is the probability of achieving a live birth within a single cycle. In ecology , fecundity is a measure of the reproductive capacity of an individual or population, typically restricted to

1680-458: 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. Hormonal contraceptives prevent pregnancy by inhibiting the secretion of the hormones, FSH, LH and GnRH. Hormonal contraception that contains estrogen, such as combined oral contraceptive pills (COCPs), stop

1764-416: Is initiated each month by falling levels of estrogen and progesterone and the release of prostaglandins , which constrict the spiral arteries . 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,

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1848-466: Is low. Occasionally high density can stimulate the production of offspring, particularly in plant species, because if there are more plants, there is food to lure pollinators, who will then spread that plant's pollen and allow for more reproduction. There are many different hypotheses to explain the relationship between latitude and fecundity. One claimed that fecundity increases predictably with increasing latitude. Reginald Morean proposed this hypothesis,

1932-714: Is not considered a disease, it can be a sign of an underlying condition such as polycystic ovary syndrome . Anovulatory cycles or short luteal phases are normal when women are under stress or athletes increasing the intensity of training. These changes are reversible as the stressors decrease or, in the case of the athlete, as she adapts to the training. Although a normal and natural process, some women experience premenstrual syndrome with symptoms that may include acne , tender breasts , and tiredness . More severe symptoms that affect daily living are classed as premenstrual dysphoric disorder and are experienced by 3 to 8% of women. Dysmenorrhea (menstrual cramps or period pain)

2016-449: Is the first part of the ovarian cycle and it ends with the completion of the antral follicles . 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. 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. Through

2100-603: Is unproven. Australian researchers have found evidence to suggest obesity may cause subtle damage to sperm and prevent a healthy pregnancy. They reported fertilization was 40% less successful when the father was overweight. The American Fertility Society recommends an age limit for sperm donors of 50 years or less, and many fertility clinics in the United Kingdom will not accept donations from men over 40 or 45 years of age. The French pronatalist movement from 1919 to 1945 failed to convince French couples they had

2184-575: Is usually between 45 and 55 years of age. Between menarche and menopause the ovaries regularly alternate between luteal and follicular phases during the monthly menstrual cycle. 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

2268-627: The Citizendium article " Fertility (demography) ", which is licensed under the Creative Commons Attribution-ShareAlike 3.0 Unported License but not under the GFDL . Fecundity Fecundity is defined in two ways; in human demography , it is the potential for reproduction of a recorded population as opposed to a sole organism , while in population biology , it is considered similar to fertility ,

2352-518: The Easterlin hypothesis to account for the Baby Boom. Bongaarts proposed a model where the total fertility rate of a population can be calculated from four proximate determinants and the total fecundity (TF). The index of marriage (Cm), the index of contraception (Cc), the index of induced abortion (Ca) and the index of postpartum infecundability (Ci). These indices range from 0 to 1. The higher

2436-470: The rhythm method of contraception. The average age of menarche in the United States is about 12.5 years. In postmenarchal girls, about 80% of the cycles are anovulatory (ovulation does not actually take place) in the first year after menarche, 50% in the third and 10% in the sixth year. Menopause occurs during a woman's midlife between ages 48 and 55. During menopause, hormonal production by

2520-412: The vagina . A woman's menstrual cycle begins, as arbitrarily assigned, with menses. Next is the follicular phase where estrogen levels build as an ovum matures (due to the follicular stimulating hormone, or FSH) within the ovary. When estrogen levels peak, it spurs a surge of luteinizing hormone (LH) which completes maturation and enables the ovum to break through the ovary wall. This is ovulation. During

2604-509: The 1970s. In mid-2018, there was a bill introduced to legalize single women and lesbian couples to get fertility treatment. At the beginning of 2020, the Senate approved the bill 160 votes to 116. They are a step closer to legalizing fertility treatments for all women regardless of sexual orientation or marital status. Soon there will be no reason for lesbian couples or single women to travel to be able to start their own family. South Korea has

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2688-412: The Baby Boom years (1946–1964), women married earlier and had their babies sooner; the number of children born to mothers after age 35 did not increase. After 1960, new methods of contraception became available, ideal family size fell, from 3 to 2 children. Couples postponed marriage and first births, and they sharply reduced the number of third and fourth births. This article incorporates material from

2772-687: The Fx is 16, and the mx is 1.336. Infecundity is a term meaning "inability to conceive after several years of exposure to the risk of pregnancy." This usage is prevalent in medicine, especially reproductive medicine , and in demographics . Infecundity would be synonymous with infertility , but in demographic and medical use fertility (and thus its antonym infertility ) may refer to quantity and rates of offspring produced, rather than any physiological or other limitations on reproduction. Additionally, social trends and societal norms may influence fecundity, though this influence tends to be temporary. Indeed, it

2856-424: The average person. The proliferative phase is the second phase of the uterine cycle when estrogen causes the lining of the uterus to grow and proliferate. The latter part of the follicular phase overlaps with the proliferative phase of the uterine cycle. 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

2940-605: The corpus luteum atrophies. Falling levels of progesterone trigger menstruation and the beginning of the next cycle. From the time of ovulation until progesterone withdrawal has caused menstruation to begin, the process typically takes about two weeks. 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). The uterine cycle has three phases: menses, proliferative and secretory. Menstruation (also called menstrual bleeding, menses or

3024-427: 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

3108-437: 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). 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

3192-500: The dead offspring. It also allows the organism to care for its offspring, as they will be alive during their development. There are a multitude of factors that potentially affect the rates of fecundity. For example: ontogeny , population density and latitude . Ontogeny Fecundity in iteroparous organisms often increases with age but can decline at older ages. Several hypotheses have been proposed to explain this relationship. For species with declining growth rates after maturity,

3276-606: The decline begins around the age of 32, and becomes precipitous at age 37. For men, potency and sperm quality begins to decline around the age of 40. Even if an older couple does manage to conceive a child, the pregnancy will be increasingly difficult for the mother, and carries a higher risk of birth defects and genetic disorders for the child. Pregnancy rates for sexual intercourse are highest when it occurs every 1 or 2 days, or every 2 or 3 days. Studies have found no significant difference between different sex positions and pregnancy rate, as long as it results in ejaculation into

3360-473: The declining fertility rate. From 1800 to 1940, fertility fell in the US. There was a marked decline in fertility in the early 1900s, associated with improved contraceptives, greater access to contraceptives and sexuality information and the "first" sexual revolution in the 1920s. After 1940 fertility suddenly started going up again, reaching a new peak in 1957. After 1960, fertility started declining rapidly. In

3444-493: The definition of fertility is "the capacity to establish a clinical pregnancy." Women have hormonal cycles which determine when they can achieve pregnancy . The cycle is approximately twenty-eight days long, with a fertile period of five days per cycle, but can deviate greatly from this norm. Men are fertile continuously, but their sperm quality is affected by their health, frequency of ejaculation, and environmental factors. Fertility declines with age in both sexes. For women,

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3528-490: The development of the dominant follicle and the mid-cycle LH surge and thus ovulation. Sequential dosing and discontinuation of the COCP can mimic the uterine cycle and produce bleeding that resembles a period. In some cases, this bleeding is lighter. 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

3612-436: 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

3696-474: The expense of survival. In semelparous species, age is frequently a poor predictor of fecundity. In these cases, size is likely a better predictor. Population density is often observed to negatively affect fecundity, making fecundity density-dependent. The reasoning behind this observation is that once an area is overcrowded, fewer resources are available for each individual. Thus there may be insufficient energy to reproduce in high numbers when offspring survival

3780-429: The explanation being that there is higher mortality in seasonal environments. A different hypothesis by David Lack attributed the positive relationship to the change in daylight hours found with changing latitudes. These differing daylight hours, in turn, change the hours in which a parent can collect food. He also accounts for a drop in fecundity at the poles due to their extreme amounts of day lengths, which can exhaust

3864-422: The first few days of menstruation some women experience period pain that can spread from the abdomen to the back and upper thighs. The menstrual cycle can be modified by hormonal birth control . The menstrual cycle encompasses the ovarian and uterine cycles. The ovarian cycle describes changes that occur in the follicles of the ovary , whereas the uterine cycle describes changes in the endometrial lining of

3948-496: The following years. Anovulatory cycles are often overtly identical to normally ovulatory cycles. Any alteration to balance of hormones can lead to anovulation. Stress, anxiety and eating disorders can cause a fall in GnRH, and a disruption of the menstrual cycle. Chronic anovulation occurs in 6–15% of women during their reproductive years. Around menopause, hormone feedback dysregulation leads to anovulatory cycles. Although anovulation

4032-451: The increase in luteinizing hormone, known as the LH surge, the dominant follicle releases an oocyte , in an event called ovulation . 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

4116-483: The index, the higher it will make the TFR, for example a population where there are no induced abortions would have a Ca of 1, but a country where everybody used infallible contraception would have a Cc of 0. TFR = TF × Cm × Ci × Ca × Cc These four indices can also be used to calculate the total marital fertility (TMFR) and the total natural fertility (TN). TFR = TMFR × Cm TMFR = TN × Cc × Ca TN = TF × Ci In medicine,

4200-475: 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

4284-492: 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,

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4368-483: The lowest fertility rate in the world at 0.78. A variety of explanations have been proposed, ranging from investment in education to birth control , abortion , a decline in the marriage rate, divorce , female participation in the labor force, and the 1997 Asian financial crisis . After being legal from the 1960s to the 1980s, abortion was again made illegal in South Korea in the early 2000s in an attempt to reverse

4452-420: The luteal phase following ovulation LH and FSH cause the post-ovulation ovary to develop into the corpus luteum which produces progesterone. The production of progesterone inhibits the LH and FSH hormones which (in a cycle without pregnancy) causes the corpus luteum to atrophy, and menses to begin the cycle again. Peak fertility occurs during just a few days of the cycle: usually two days before and two days after

4536-454: The menstrual cycle but do not translate into measurable changes in intellectual achievement – including academic performance, problem-solving, and memory. Improvements in spatial reasoning 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

4620-401: 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

4704-545: The natural capability to produce offspring, measured by the number of gametes (eggs), seed set, or asexual propagules. Human demography considers only human fecundity, at its culturally differing rates, while population biology studies all organisms . The term fecundity in population biology is often used to describe the rate of offspring production after one time step (often annual). In this sense, fecundity may include both birth rates and survival of young to that time step. While levels of fecundity vary geographically, it

4788-413: 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. Measured from the first day of one menstruation to the first day of

4872-426: 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

4956-714: The number of seminiferous tubules in the testes containing mature sperm: Decline in male fertility is influenced by many factors, including lifestyle, environment and psychological factors. Some research suggests increased risks for health problems for children of older fathers, but no clear association has been proven. A large scale study in Israel suggested that the children of men 40 or older were 5.75 times more likely than children of men under 30 to have an autism spectrum disorder , controlling for year of birth, socioeconomic status, and maternal age. Increased paternal age has been suggested to correlate with schizophrenia but it

5040-403: The other 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 0.1 mm (0.0039 in), 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,

5124-579: The other hand, follows the same people over a period of decades. Both period and cohort measures are widely used. A parent's number of children strongly correlates with the number of children that each person in the next generation will eventually have. Factors generally associated with increased fertility include religiosity , intention to have children, and maternal support. Factors generally associated with decreased fertility include wealth , education, female labor participation , urban residence, cost of housing, intelligence , increased female age and (to

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5208-490: The ovaries is reduced, eventually causing a permanent cessation of the creation of the uterine lining (period). This is considered the end of the fertile phase of a woman's life. The predicted effect of age on female fertility in women trying to get pregnant, without using fertility drugs or in vitro fertilization: Studies of couples trying to conceive have yielded better results: one 2004 study of 770 European women found that 82% of 35- to 39-year-old women conceived within

5292-443: The ovulation date. This fertile window varies from woman to woman, just as the ovulation date often varies from cycle to cycle for the same woman. The ovule is usually capable of being fertilized for up to 48 hours after it is released from the ovary . Sperm survive inside the uterus between 48 and 72 hours on average, with the maximum being 120 hours (5 days). These periods and intervals are important factors for couples using

5376-481: 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 seizures occur more frequently at certain phases of their menstrual cycle. This catamenial epilepsy may be due to

5460-603: The parent. Fecundity intensity due to seasonality is a hypothesis proposed by Phillip Ashmole. He suggests latitude affects fecundity due to seasonality increasing with increasing latitudes. This theory relies on the mortality concept proposed by Moreau but focuses on how seasonality affects mortality and, in turn, population densities. Thus in places with higher mortality, there is more food availability, leading to higher fecundity. Another hypothesis claims that seasonality affects fecundity due to varying lengths of breeding seasons. This idea suggests that shorter breeding seasons select

5544-424: The patterns of birth amongst individuals of different ages in a population. These are typically found in life tables under the columns Fx and mx. Fx lists the total number of young produced by each age class, and mx is the mean number of young produced, found by finding the number of young produced per surviving individual. For example, if you have 12 individuals in an age class and they produced 16 surviving young,

5628-422: The peak in LH surge. Typically only one of the 15–20 stimulated follicles reaches full maturity, and just one egg is released. 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. LH initiates ovulation at around day 14 and stimulates the formation of the corpus luteum. Following further stimulation by LH,

5712-440: 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. Around day fourteen, the egg is released from the ovary. 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

5796-438: The reproductive individuals. It can be equally applied to sexual and asexual reproduction, as the purpose of fecundity is to measure how many new individuals are being added to a population. Fecundity may be defined differently for different ecological studies to explain the specific data the study examined. For example, some studies use apparent fecundity to describe that their data looks at a particular moment in time rather than

5880-414: 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

5964-414: The rest of the pregnancy, the placenta secretes high levels of these hormones – along with human chorionic gonadotropin (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. Lasting about 14 days, the luteal phase is the final phase of the ovarian cycle and it corresponds to

6048-408: The result is fraternal twins . 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 ,

6132-517: 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. 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

6216-529: The species' entire life span. In other studies, these definitions are changed to better quantify fecundity for the organism in question. This need is particularly true for modular organisms , as their modular organization differs from the more typical unitary organism, in which fecundity is best defined through a count of offspring. Parity is the organization of fecundity into two distinct types, semelparity, and iteroparity. Semelparity occurs when an organism reproduces only once in its lifetime, with death being

6300-548: The stratum basalis, is left in place. An enzyme called plasmin breaks up the blood clots 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

6384-439: 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 eggs 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

6468-435: The suggestion is that as the organism's growth rate decreases, more resources can be allocated to reproduction. Other possible explanations exist for this pattern for organisms that do not grow after maturity. These explanations include: increased competence of older individuals; less fit individuals have already died off; or since life expectancy decreases with age, older individuals may allocate more resources to reproduction at

6552-422: The thickened lining provides nutrients to a successfully implanted embryo. If implantation does not occur, the lining breaks down and blood is released. Triggered by falling progesterone levels, menstruation (a "period", in common parlance) 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

6636-474: 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. Beginning at puberty, these mature to primary follicles independently of the menstrual cycle. The development of the egg is called oogenesis and only one cell survives the divisions to await fertilization. The other cells are discarded as polar bodies , which cannot be fertilized. The follicular phase

6720-443: The uterus and the cervical mucus thickens. In early pregnancy, progesterone also increases blood flow and reduces the contractility of the smooth muscle in the uterus and raises basal body temperature . If pregnancy does not occur the ovarian and uterine cycles start over again. Only two-thirds of overtly normal menstrual cycles are ovulatory, that is, cycles in which ovulation occurs. The other third lack ovulation or have

6804-648: 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 tiredness . More severe symptoms that affect daily living are classed as premenstrual dysphoric disorder , and are experienced by 3–8% of women. During

6888-466: The uterus with the spiral arterioles. As estrogen levels increase, cells in the cervix produce a type of cervical mucus that has a higher pH and is less viscous 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

6972-448: The uterus. Both cycles can be divided into phases. The ovarian cycle consists of alternating follicular and luteal phases , and the uterine cycle consists of the menstrual phase , the proliferative phase, and the secretory phase. 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

7056-404: The widest section of the fallopian tubes. A fertilized egg 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

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