Fertility

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Fertility in colloquial terms refers the ability to have offspring. In demographic contexts, fertility refers to the actual production of offspring, rather than the physical capability to reproduce, which is termed fecundity. [1] [2] The fertility rate is the average number of children born during an individual's lifetime. In medicine, fertility refers to the ability to have children, and infertility refers to difficulty in reproducing naturally. [3] In general, infertility or subfertility [4] in humans is defined as not being able to conceive a child after one year (or longer) of unprotected sex. [5] The antithesis of fertility is infertility, while the antithesis of fecundity is sterility.

Contents

Demography

In demographic contexts, fertility refers to the actual production of offspring, rather than the physical capability to produce which is termed fecundity. [1] [2] 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 the other hand, follows the same people over a period of decades. Both period and cohort measures are widely used. [6]

Period measures

Cohort measures

Countries by fertility rate as of 2020 Total Fertility Rate Map by Country.svg
Countries by fertility rate as of 2020

Social and economic determinants of fertility

A parent's number of children strongly correlates with the number of children that each person in the next generation will eventually have. [8] Factors generally associated with increased fertility include religiosity, [9] intention to have children, [10] and maternal support. [11] Factors generally associated with decreased fertility include wealth, education, [12] [13] female labor participation, [14] urban residence, [15] cost of housing, [16] [17] intelligence, increased female age and (to 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: [18] [19] 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), [20] Mincer (1963), [21] and Easterlin (1969). [22] The latter developed the Easterlin hypothesis to account for the Baby Boom.

Bongaarts' model of components of fertility

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 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

Intercourse
The first step is sexual intercourse, and an examination of the average age at first intercourse, the average frequency outside marriage, and the average frequency inside.
Conception
Certain physical conditions may make it impossible for a woman to conceive. This is called "involuntary infecundity." If the woman has a condition making it possible, but unlikely to conceive, this is termed "subfecundity." Venereal diseases (especially gonorrhea, syphilis, and chlamydia) are common causes. Nutrition is a factor as well: women with less than 20% body fat may be subfecund, a factor of concern for athletes and people susceptible to anorexia. Demographer Ruth Frisch has argued that "It takes 50,000 calories to make a baby". There is also subfecundity in the weeks following childbirth, and this can be prolonged for a year or more through breastfeeding. A furious political debate raged in the 1980s over the ethics of baby food companies marketing infant formula in developing countries. A large industry has developed to deal with subfecundity in women and men. An equally large industry has emerged to provide contraceptive devices designed to prevent conception. Their effectiveness in use varies. On average, 85% of married couples using no contraception will have a pregnancy in one year. The rate drops to the 20% range when using withdrawal, vaginal sponges, or spermicides. (This assumes the partners never forget to use the contraceptive.) The rate drops to only 2 or 3% when using the pill or an IUD, and drops to near 0% for implants and 0% for tubal ligation (sterilization) of the woman, or a vasectomy for the man.
Gestation
After a fetus is conceived, it may or may not survive to birth. "Involuntary fetal mortality" involves natural abortion, miscarriages and stillbirth (a fetus born dead). Human intervention intentionally causing abortion of the fetus is called "therapeutic abortion".

In medicine

In medicine, the definition of fertility is "the capacity to establish a clinical pregnancy." [23]

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. [24]

Fertility declines with age in both sexes. For women, 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. [25]

Pregnancy rates for sexual intercourse are highest when it occurs every 1 or 2 days, [26] or every 2 or 3 days. [27] Studies have found no significant difference between different sex positions and pregnancy rate, as long as it results in ejaculation into the vagina. [28]

Menstrual cycle

Chance of fertilization by menstrual cycle day relative to ovulation Pregnancy chance by day near ovulation.jpg
Chance of fertilization by menstrual cycle day relative to ovulation

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. [30] This is ovulation. During 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 the ovulation date. [31] This fertile window varies from woman to woman, just as the ovulation date often varies from cycle to cycle for the same woman. [32] 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 the rhythm method of contraception.

Female fertility

The average age of menarche in the United States is about 12.5 years. [33] 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. [34]

Menopause occurs during a woman's midlife between ages 48 and 55. [35] [36] During menopause, hormonal production by 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: [37]

[37]

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 a year, [38] while a study in 2013 of 2,820 Danish women saw 78% of 35- to 40-year-olds conceive within a year. [39]

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. [40]

Successful pregnancies facilitated by fertility treatment have been documented in women as old as 67. [41]

Male fertility

Some research suggests that older males have decreased semen volume, sperm motility, and impaired sperm morphology. [42] 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%. [42] 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 the number of seminiferous tubules in the testes containing mature sperm: [42]

Decline in male fertility is influenced by many factors, including lifestyle, environment and psychological factors. [44]

Some research suggests increased risks for health problems for children of older fathers, but no clear association has been proven. [45] 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. [46] Increased paternal age has been suggested to correlate with schizophrenia but it is unproven. [47] [48] [49] [50] [51]

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. [52]

The American Fertility Society recommends an age limit for sperm donors of 50 years or less, [53] and many fertility clinics in the United Kingdom will not accept donations from men over 40 or 45 years of age. [54]

France

The French pronatalist movement from 1919 to 1945 failed to convince French couples they had 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 the 1970s. [55] 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. [56]

Korea

Total fertility rate in Korea Total Fertility Rate (TFR) in Korea.svg
Total fertility rate in Korea

South Korea has the lowest fertility rate in the world at 0.78. [57] A variety of explanations have been proposed, ranging from investment in education [58] to birth control, abortion, a decline in the marriage rate, divorce, female participation in the labor force, and the 1997 Asian financial crisis. [59] 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 the declining fertility rate. [60]

United States

United States crude birth rate (births per 1000 population); Baby Boom years in red. US Birth Rates.svg
United States crude birth rate (births per 1000 population); Baby Boom years in red.

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.

Post-WWII

After 1940 fertility suddenly started going up again, reaching a new peak in 1957. After 1960, fertility started declining rapidly. In 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. [62]

Sexual revolution

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. [63]

See also

Further reading

Related Research Articles

<span class="mw-page-title-main">Fertility awareness</span> Methods to determine menstrual phases

Fertility awareness (FA) refers to a set of practices used to determine the fertile and infertile phases of a woman's menstrual cycle. Fertility awareness methods may be used to avoid pregnancy, to achieve pregnancy, or as a way to monitor gynecological health.

Calendar-based methods are various methods of estimating a woman's likelihood of fertility, based on a record of the length of previous menstrual cycles. Various methods are known as the Knaus–Ogino method and the rhythm method. The standard days method is also considered a calendar-based method, because when using it, a woman tracks the days of her menstrual cycle without observing her physical fertility signs. The standard days method is based on a fixed formula taking into consideration the timing of ovulation, the functional life of the sperm and the ovum, and the resulting likelihood of pregnancy on particular days of the menstrual cycle. These methods may be used to achieve pregnancy by timing unprotected intercourse for days identified as fertile, or to avoid pregnancy by avoiding unprotected intercourse during fertile days.

<span class="mw-page-title-main">In vitro fertilisation</span> Assisted reproductive technology procedure

In vitro fertilisation (IVF) is a process of fertilisation in which an egg is combined with sperm in vitro. The process involves monitoring and stimulating a woman's ovulatory process, then removing an ovum or ova from her ovaries and enabling a man's sperm to fertilise them in a culture medium in a laboratory. After a fertilised egg (zygote) undergoes embryo culture for 2–6 days, it is transferred by catheter into the uterus, with the intention of establishing a successful pregnancy.

Reproductive technology encompasses all current and anticipated uses of technology in human and animal reproduction, including assisted reproductive technology (ART), contraception and others. It is also termed Assisted Reproductive Technology, where it entails an array of appliances and procedures that enable the realization of safe, improved and healthier reproduction. While this is not true of all people, for an array of married couples, the ability to have children is vital. But through the technology, infertile couples have been provided with options that would allow them to conceive children.

<span class="mw-page-title-main">Infertility</span> Inability to reproduce by natural means

Infertility is the inability of a couple to reproduce by natural means. It is usually not the natural state of a healthy adult. Exceptions include children who have not undergone puberty, which is the body's start of reproductive capacity. It is also a normal state in women after menopause.

<span class="mw-page-title-main">Artificial insemination</span> Pregnancy through in vivo fertilization

Artificial insemination is the deliberate introduction of sperm into a female's cervix or uterine cavity for the purpose of achieving a pregnancy through in vivo fertilization by means other than sexual intercourse. It is a fertility treatment for humans, and is a common practice in animal breeding, including dairy cattle and pigs.

<span class="mw-page-title-main">Assisted reproductive technology</span> Methods to achieve pregnancy by artificial or partially artificial means

Assisted reproductive technology (ART) includes medical procedures used primarily to address infertility. This subject involves procedures such as in vitro fertilization (IVF), intracytoplasmic sperm injection (ICSI), cryopreservation of gametes or embryos, and/or the use of fertility medication. When used to address infertility, ART may also be referred to as fertility treatment. ART mainly belongs to the field of reproductive endocrinology and infertility. Some forms of ART may be used with regard to fertile couples for genetic purpose. ART may also be used in surrogacy arrangements, although not all surrogacy arrangements involve ART. The existence of sterility will not always require ART to be the first option to consider, as there are occasions when its cause is a mild disorder that can be solved with more conventional treatments or with behaviors based on promoting health and reproductive habits.

Fertility medications, also known as fertility drugs, are medications which enhance reproductive fertility. For women, fertility medication is used to stimulate follicle development of the ovary. There are very few fertility medication options available for men.

<span class="mw-page-title-main">Female infertility</span> Diminished or absent ability of a female to achieve conception

Female infertility refers to infertility in women. It affects an estimated 48 million women, with the highest prevalence of infertility affecting women in South Asia, Sub-Saharan Africa, North Africa/Middle East, and Central/Eastern Europe and Central Asia. Infertility is caused by many sources, including nutrition, diseases, and other malformations of the uterus. Infertility affects women from around the world, and the cultural and social stigma surrounding it varies.

<span class="mw-page-title-main">Lactational amenorrhea</span> Post-partum infertility due to breast feeding

Lactational amenorrhea, also called postpartum infertility, is the temporary postnatal infertility that occurs when a woman is amenorrheic and fully breastfeeding.

<span class="mw-page-title-main">Human reproduction</span> Procreative biological processes of humanity

Human reproduction is sexual reproduction that results in human fertilization to produce a human offspring. It typically involves sexual intercourse between a sexually mature human male and female. During sexual intercourse, the interaction between the male and female reproductive systems results in fertilization of the ovum by the sperm to form a zygote. While normal cells contain 46 chromosomes, gamete cells only contain 23 single chromosomes, and it is when these two cells merge into one zygote cell that genetic recombination occurs and the new zygote contains 23 chromosomes from each parent, giving it 46 chromosomes. The zygote then undergoes a defined development process that is known as human embryogenesis, and this starts the typical 9-month gestation period that is followed by childbirth. The fertilization of the ovum may be achieved by artificial insemination methods, which do not involve sexual intercourse. Assisted reproductive technology also exists.

Childlessness is the state of not having children. Childlessness may have personal, social or political significance.

Natural fertility is the fertility that exists without birth control or other medical interventions. The control is the number of children birthed to the parents and is modified as the number of children reaches the maximum. Natural fertility tends to decrease as a society modernizes. Women in a pre-modernized society typically have given birth to a large number of children by the time they are 50 years old, while women in post-modernized society only bear a small number by the same age. However, during modernization natural fertility rises, before family planning is practiced.

Unexplained infertility is infertility that is idiopathic in the sense that its cause remains unknown even after an infertility work-up, usually including semen analysis in the man and assessment of ovulation and fallopian tubes in the woman. It is usually an exercise in excluding all possible causes before making a diagnosis, however the age of the female partner as well as the duration of infertility are often the most scrutinized characteristics of any infertility case.

Infertility in polycystic ovary disease (PCOS) is a hormonal imbalance in women that is thought to be one of the leading causes of female infertility. Polycystic ovary syndrome causes more than 75% of cases of anovulatory infertility.

Female fertility is affected by age and is a major fertility factor for women. A woman's fertility is in generally good quality from the late teens to early thirties, although it declines gradually over time. Around 35, fertility is noted to decline at a more rapid rate. At age 45, a woman starting to try to conceive will have no live birth in 50–80 percent of cases. Menopause, or the cessation of menstrual periods, generally occurs in the 40s and 50s and marks the cessation of fertility, although age-related infertility can occur before then. The relationship between age and female fertility is sometimes referred to as a woman's "biological clock."

Endometriosis and its complications are a major cause of female infertility. Endometriosis is a dysfunction characterized by the migration of endometrial tissue to areas outside of the endometrium of the uterus. The most common places to find stray tissue are on ovaries and fallopian tubes, followed by other organs in the lower abdominal cavity such as the bladder and intestines. Typically, the endometrial tissue adheres to the exteriors of the organs, and then creates attachments of scar tissue called adhesions that can join adjacent organs together. The endometrial tissue and the adhesions can block a fallopian tube and prevent the meeting of ovum and sperm cells, or otherwise interfere with fertilization, implantation and, rarely, the carrying of the fetus to term.

Obesity is defined as an abnormal accumulation of body fat, usually 20% or more over an individual's ideal body weight. This is often described as a body mass index (BMI) over 30. However, BMI does not account for whether the excess weight is fat or muscle, and is not a measure of body composition. For most people, however, BMI is an indication used worldwide to estimate nutritional status. Obesity is usually the result of consuming more calories than the body needs and not expending that energy by doing exercise. There are genetic causes and hormonal disorders that cause people to gain significant amounts of weight but this is rare. People in the obese category are much more likely to suffer from fertility problems than people of normal healthy weight.

Human reproductive ecology is a subfield in evolutionary biology that is concerned with human reproductive processes and responses to ecological variables. It is based in the natural and social sciences, and is based on theory and models deriving from human and animal biology, evolutionary theory, and ecology. It is associated with fields such as evolutionary anthropology and seeks to explain human reproductive variation and adaptations. The theoretical orientation of reproductive ecology applies the theory of natural selection to reproductive behaviors, and has also been referred to as the evolutionary ecology of human reproduction.

<span class="mw-page-title-main">Use of assisted reproductive technology by LGBT people</span>

Lesbian, gay, bisexual, and transgender people people wishing to have children may use assisted reproductive technology. In recent decades, developmental biologists have been researching and developing techniques to facilitate same-sex reproduction.

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