Fertility

"Fertile" redirects here. For other uses, see Fertile (disambiguation).

Fertility is the natural capability to produce offspring. As a measure, fertility rate is the number of offspring born per mating pair, individual or population. Fertility differs from fecundity, which is defined as the potential for reproduction (influenced by gamete production, fertilization and carrying a pregnancy to term) . A lack of fertility is infertility while a lack of fecundity would be called sterility.

Human fertility depends on factors of nutrition, sexual behavior, consanguinity, culture, instinct, endocrinology, timing, economics, way of life, and emotions.

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.[3]

Period measures

Cohort measures

Social determinants of fertility

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:[5][6]

Economic analysis of fertility

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),[7] Mincer (1963),[8] and Easterlin (1969).[9] 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".

Human fertility

Men and women have hormonal cycles which determine both when a woman can achieve pregnancy and when a man is most virile. The female cycle is approximately twenty-eight days long, but can deviate greatly from this "norm". The male cycle is also variable. Men can ejaculate and produce sperm at any time of the month, but their sperm quality dips occasionally, which scientists guess is in relation to their internal cycle.

Furthermore, age also plays an equally significant role for both men and women.

Menstrual cycle

Main article: Menstrual cycle

A woman's menstrual cycle begins, as it has been 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 finishes the ovum and enables it to break through the ovary wall. This is ovulation. During the luteal phase, which follows 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.[10] This fertile window varies from woman to woman, just as the ovulation date often varies from cycle to cycle for the same woman.[11] 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 to 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

Further information: Female infertility

The average age of menarche in the United States is about 12.5 years.[12] In postmenarchal girls, about 80% of the cycles are anovulatory in the first year after menarche, 50% in the third and 10% in the sixth year.[13][14]

Women's fertility peaks in the early 20s, and drops considerably after age 35.[15] Menopause typically occurs during a women's midlife (usually between ages 45 and 55 [16][17]). During menopause, hormonal production by the ovaries is reduced, eventually causing a permanent cessation of the primary function of the ovaries, particularly the creation of the uterine lining (period). This is considered the end of the fertile phase of a woman's life.

According to a computer simulation run by Henri Leridon, PhD, an epidemiologist with the French Institute of Health and Medical Research, of women trying to get pregnant, without using fertility drugs or in vitro fertilization [18]

[18]

Studies of actual couples trying to conceive have come up with higher results: one 2004 study of 770 European women found that 82% of 35-to-39-year-old women conceived within a year,[19] while another in 2013 of 2,820 Danish women saw 78% of 35-to-40-year-olds conceive within a year.[20]

The use of fertility drugs and/or invitro fertilization can increase the chances of becoming pregnant at a later age.[21] Successful pregnancies facilitated by fertility treatment have been documented in women as old as 67.[22] Studies since 2004 now show 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, US, say that if eggs are newly created each month in humans as well, all current theories about the aging of the female reproductive system will have to be overhauled, although at this time this is simply conjecture.[23][24] Furthermore, holistic fertility treatments (those that encompass physical, intellectual, emotional, and spiritual healing) such as homeopathy, herbs, and craniosacral therapy have been praised by a number of experts and women.[25]

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".[26] Birth defects, especially those involving chromosome number and arrangement, also increase with the age of the mother. According to the March of Dimes, "At age 25, a woman has about a 1-in-1,250 chance of having a baby with Down syndrome; at age 30, a 1-in-1,000 chance; at age 35, a 1-in-400 chance; at age 40, a 1-in-100 chance; and at 45, a 1-in-30 chance."[27]

Male fertility

Evidence shows that increased male age is associated with a decline in semen volume, sperm motility, and sperm morphology.[28] 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%.[28]

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. However, an even larger difference is seen in how many of the seminiferous tubules in the testes contain mature sperm;

Decline in male fertility is influenced by many factors, including lifestyle, environment and psychological factors.[30]

Research shows increased risks for health problems for children of older fathers. A large scale Israeli study found 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.[31] Increased paternal age has also been correlated to schizophrenia in numerous studies.[32][33][34]

Australian researchers have found evidence to suggest overweight obesity may cause subtle damage to sperm and prevent a healthy pregnancy. They say fertilization was 40% less likely to succeed when the father was overweight.[35]

The American Fertility Society recommends an age limit for sperm donors of 50 years or less,[36] and many fertility clinics in the United Kingdom will not accept donations from men over 40 or 45 years of age.[37] In part because of this fact, more women are now using a take-home baby rate calculator to estimate their chances of success following invitro fertilization.[38] Campagne DM Can Male Fertility Be Improved Prior to Assisted Reproduction through The Control of Uncommonly Considered Factors? PMCID: PMC3850314

Historical trends by country

Main article: Historical demography

France

The French pronatalist movement from 1919-1945 failed to convince French couples of having 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.[39]

United States

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.

United States crude birth rate (births per 1000 population); Baby Boom years in red.[40]

Post-WW II

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.

Easterlin model

American economist Richard Easterlin developed a theory (the Easterlin hypothesis) to account for the Baby Boom. He assumes first that young couples try to achieve a standard of living equal to or better than they had when they grew up. This is called "relative status"; in other words, young men in one cohort compare themselves now to where their own fathers in a previous cohort had been. Second, Easterlin assumes that when jobs are plentiful, it will be easier to marry young and have more children and still match that standard of living. But when jobs are scarce, couples who try to keep that standard of living will wait to get married and have fewer children. For Easterlin, the size of the cohort is a critical determinant of how easy it is to get a good job. A small cohort means less competition, a large cohort means more competition to worry about. The assumptions blend economics and sociology, and Easterlin did not rely on surveys or interviews asking people what really motivated them.[41]

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.

Infertility

Main article: Infertility

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.[42]

See also

Footnotes

  1. ↑ http://www.gfmer.ch/Books/Reproductive_health/The_demography_of_fertility_and_infertility.html
  2. ↑ http://www.enotes.com/public-health-encyclopedia/fecundity-fertility
  3. ↑ For detailed discussions of each measure see Paul George Demeny and Geoffrey McNicoll, Encyclopedia of Population (2003)
  4. ↑ Another way of doing it is to add up the ASFR for age 10-14, 15-19, 20-24, etc., and multiply by 5 (to cover the 5 year interval).
  5. ↑ Bongaarts, John (1978). "A Framework for Analyzing the Proximate Determinants of Fertility". Population and Development Review 4 (1): 105–132. JSTOR 1972149.
  6. ↑ Stover, John (1998). "Revising the Proximate Determinants of Fertility Framework: What Have We Learned in the past 20 Years?". Studies in Family Planning 29 (3): 255–267. JSTOR 172272.
  7. ↑ Becker, Gary S. 1960. "An Economic Analysis of Fertility." In National Bureau Committee for Economic Research, Demographic and Economic Change in Developed Countries, a Conference of the Universities. Princeton, N.J.: Princeton University Press
  8. ↑ Mincer, Jacob. 1963. "Market Prices, Opportunity Costs, and Income Effects," in C. Christ (ed.) Measurement in Economics. Stanford, CA: Stanford University Press
  9. ↑ Easterlin, Richard A. (1975). Studies in Family Planning 6 (3): 54–63. doi:10.2307/1964934. Missing or empty |title= (help)
  10. ↑ http://www.duofertility.com/en/my-body/my-cycle/my-fertile-period.html
  11. ↑ Creinin MD, Keverline S, Meyn LA (October 2004). "How regular is regular? An analysis of menstrual cycle regularity". Contraception 70 (4): 289–92. doi:10.1016/j.contraception.2004.04.012. PMID 15451332.
  12. ↑ Anderson SE, Dallal GE, Must A (April 2003). "Relative weight and race influence average age at menarche: results from two nationally representative surveys of US girls studied 25 years apart". Pediatrics 111 (4 Pt 1): 844–50. doi:10.1542/peds.111.4.844. PMID 12671122.
  13. ↑ Apter D (February 1980). "Serum steroids and pituitary hormones in female puberty: a partly longitudinal study". Clin. Endocrinol. (Oxf) 12 (2): 107–20. doi:10.1111/j.1365-2265.1980.tb02125.x. PMID 6249519.
  14. ↑ Apter D (February 1980). "Serum steroids and pituitary hormones in female puberty: a partly longitudinal study". Clin. Endocrinol. (Oxf) 12 (2): 107–20. doi:10.1111/j.1365-2265.1980.tb02125.x. PMID 6249519.
  15. ↑ http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002173/
  16. ↑ http://www.nlm.nih.gov/medlineplus/ency/article/000894.htm
  17. ↑ http://changeoflife.com/topics/23/article/item/null/menopause-is-ovarian-failure
  18. ↑ 18.0 18.1 Leridon, H. (2004). "Can assisted reproduction technology compensate for the natural decline in fertility with age? A model assessment". Human Reproduction 19 (7): 1548–1553. doi:10.1093/humrep/deh304. PMID 15205397.
  19. ↑ http://journals.lww.com/greenjournal/Fulltext/2004/01000/Increased_Infertility_With_Age_in_Men_and_Women.10.aspxYoublacklove
  20. ↑ http://www.fertstert.org/article/S0015-0282(13)00339-7/abstract
  21. ↑ Fertility Nutraceuticals, LLC "How to improve IVF success rates with smart fertility supplement strategy' May 6, 2014
  22. ↑ "Spanish woman ' is oldest mother'". BBC News. 2006-12-30. Retrieved 2006-12-30.
  23. ↑ Couzin, Jennifer (March 2004). "Textbook Rewrite? Adult Mammals May Produce Eggs After All" (PDF). Science 303 (5664): 1593. doi:10.1126/science.303.5664.1593a. PMID 15016968.
  24. ↑ "Human Ovarian Reserve from Conception to the Menopause". PLoS ONE 5 (1): e8772. doi:10.1371/journal.pone.0008772.
  25. ↑ Indichova, Julia. "An Overview of Natural, Holistic Fertility Treatments".
  26. ↑ "Pregnancy After 35". March of Dimes. Retrieved October 30, 2014.
  27. ↑ http://www.marchofdimes.org/baby/down-syndrome.aspx
  28. ↑ 28.0 28.1 Kidd SA, Eskenazi B, Wyrobek AJ (February 2001). "Effects of male age on semen quality and fertility: a review of the literature". Fertil. Steril. 75 (2): 237–48. doi:10.1016/S0015-0282(00)01679-4. PMID 11172821.
  29. ↑ Effect of Age on Male Fertility Seminars in Reproductive Endocrinology. Volume, Number 3, August 1991. Sherman J. Silber, M.D.
  30. ↑ Campagne, DM (January 2013). "Can Male Fertility Be Improved Prior to Assisted Reproduction through The Control of Uncommonly Considered Factors?". Int J Fertility Sterility 6 (4): 214–33.
  31. ↑ Reichenberg A; Gross R; Weiser M et al. (September 2006). "Advancing paternal age and autism". Arch. Gen. Psychiatry 63 (9): 1026–32. doi:10.1001/archpsyc.63.9.1026. PMID 16953005.
  32. ↑ Malaspina D; Harlap S; Fennig S et al. (April 2001). "Advancing paternal age and the risk of schizophrenia". Arch. Gen. Psychiatry 58 (4): 361–7. doi:10.1001/archpsyc.58.4.361. PMID 11296097.
  33. ↑ Sipos A; Rasmussen F; Harrison G et al. (November 2004). "Paternal age and schizophrenia: a population based cohort study". BMJ 329 (7474): 1070. doi:10.1136/bmj.38243.672396.55. PMC 526116. PMID 15501901.
  34. ↑ Malaspina D; Corcoran C; Fahim C et al. (April 2002). "Paternal Age and Sporadic Schizophrenia: Evidence for De Novo Mutations". Am. J. Med. Genet. 114 (3): 299–303. doi:10.1002/ajmg.1701. PMC 2982144. PMID 11920852.
  35. ↑ "Obesity | Fat men linked to low fertility". Sydney Morning Herald. 18 October 2010. Retrieved 19 October 2010.
  36. ↑ Plas E, Berger P, Hermann M, Pflüger H (August 2000). "Effects of aging on male fertility?". Exp. Gerontol. 35 (5): 543–51. doi:10.1016/S0531-5565(00)00120-0. PMID 10978677.
  37. ↑ Age Limit of Sperm Donors in the United Kingdom Pdf file
  38. ↑ http://www.formyodds.com
  39. ↑ Andres Horacio Reggiani, "Procreating France: the politics of demography, 1919-1945." French Historical Studies Spring 1996 v19 n3 pp 725-54
  40. ↑ CDC Bottom of this page http://www.cdc.gov/nchs/products/vsus.htm "Vital Statistics of the United States, 2003, Volume I, Natality", Table 1-1 "Live births, birth rates, and fertility rates, by race: United States, 1909-2003."
  41. ↑ Macunovich, Diane J. (1998). "Fertility and the Easterlin hypothesis: An assessment of the literature". Journal of Population Economics 11: 1–59. CiteSeerX: 10.1.1.102.8504.
  42. ↑ Makar RS, Toth TL (2002). "The evaluation of infertility". Am J Clin Pathol. 117 Suppl: S95–103. PMID 14569805.

References

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.

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