Fertility

Fertility is the capability to produce offspring through reproduction following the onset of sexual maturity. The fertility rate is the average number of children born by a female during her lifetime and is quantified demographically. Fertility is addressed when there is a difficulty or an inability to reproduce naturally, which is referred to as infertility. Infertility is widespread, with fertility specialists available all over the world to assist mothers and couples who experience difficulties having a baby.

Human fertility depends on factors of nutrition, sexual behaviour, consanguinity, culture, instinct, endocrinology, timing, economics, personality,[1] way of life, and emotions.

Fertility differs from fecundity, which is defined as the potential for reproduction (influenced by gamete production, fertilization and carrying a pregnancy to term).[2] Where a woman or the lack of fertility is infertility while a lack of fecundity would be called sterility.

In demographic contexts, fertility refers to the actual production of offspring, rather than the physical capability to produce which is termed fecundity.[3][4] 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.[5]

A parent's number of children strongly correlates with the number of children that each person in the next generation will eventually have.[7] Factors generally associated with increased fertility include religiosity,[8] intention to have children,[9] and maternal support.[10] Factors generally associated with decreased fertility include wealth, education,[11][12] female labor participation,[13] urban residence,[14] cost of housing,[15][16] 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:[17][18] 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),[19] Mincer (1963),[20] and Easterlin (1969).[21] The latter developed 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 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

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.[22]

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.[23]

Pregnancy rates for sexual intercourse are highest when it occurs every 1 or 2 days,[24] or every 2 or 3 days.[25] Studies have found no significant difference between different sex positions and pregnancy rate, as long as it results in ejaculation into the vagina.[26]

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.[28] 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.[29] This fertile window varies from woman to woman, just as the ovulation date often varies from cycle to cycle for the same woman.[30] 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.

The average age of menarche in the United States is about 12.5 years.[31] 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.[32]

Menopause occurs during a woman's midlife between ages 48 and 55.[33][34] 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:[35]

[35]

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,[36] while a study in 2013 of 2,820 Danish women saw 78% of 35- to 40-year-olds conceive within a year.[37]

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.[38]

The use of fertility drugs and/or invitro fertilization can increase the chances of becoming pregnant at a later age.[39] Successful pregnancies facilitated by fertility treatment have been documented in women as old as 67.[40] Studies since 2004 have shown that mammals may continue to produce new eggs throughout their lives, rather than being born with a finite number as previously thought. Researchers at the Massachusetts General Hospital in Boston say that if eggs are newly created each month in humans, current theories about the aging of the female reproductive system will have to be overhauled, although as of 2010[update] this is conjecture.[41][42]

According to the March of Dimes, "about 9 percent of recognized pregnancies for women aged 20 to 24 ended in miscarriage. The risk rose to about 20 percent at age 35 to 39, and more than 50 percent by age 42".[43] Birth defects, especially those involving chromosome number and arrangement, also increase with the age of the mother. The March of Dimes reports "At age 25, your risk of having a baby with Down syndrome is 1 in 1,340. At age 30, your risk is 1 in 940. At age 35, your risk is 1 in 353. At age 40, your risk is 1 in 85. At age 45, your risk is 1 in 35."[44]

Some research suggests that older males have decreased semen volume, sperm motility, and impaired sperm morphology.[45] In studies that controlled for female age, comparisons between men under 30 and men over 50 found relative decreases in pregnancy rates between 23% and 38%.[45] 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:[45]

Decline in male fertility is influenced by many factors, including lifestyle, environment and psychological factors.[47]

Some research suggests increased risks for health problems for children of older fathers, but no clear association has been proven.[48] A large scale in Israel study 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.[49] Increased paternal age has been suggested to correlate with schizophrenia but it is unproven.[50][51][52][53][54]

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.[55]

The American Fertility Society recommends an age limit for sperm donors of 50 years or less,[56] and many fertility clinics in the United Kingdom will not accept donations from men over 40 or 45 years of age.[57]

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.[58] 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.[59]

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 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.[61]

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.[62]

Infertility primarily refers to the biological inability of a person to contribute to conception. Infertility may also refer to the state of a woman who is unable to carry a pregnancy to full term. There are many biological causes of infertility, including some that medical intervention can treat.[63]

This article incorporates material from the Citizendium article "Fertility (demography)", which is licensed under the Creative Commons Attribution-ShareAlike 3.0 Unported License but not under the GFDL.