||This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (May 2011) (Learn how and when to remove this template message)|
Natural fertility refers to the absence of parity-specific family planning, meaning that couples are not regulating their fertility based on their current parity. Family planning, on the other hand, is when couples regulate their fertility (e.g. stop having children) after reaching a specific parity. Actual natural fertility is based on cohort parity: the fraction of women in a cohort who have reached a specific parity or number of children.
Historical populations have traditionally honored the idea of fertility by displaying fertility symbols.
Natural fertility is a concept developed by the French historical demographer Louis Henry to refer to the level of fertility that would prevail in a population that makes no conscious effort to limit, regulate, or control fertility, so that fertility depends only on physiological factors affecting fecundity. In contrast, populations that practice birth control will have lower fertility levels as a result of delaying first births (a lengthened interval between menarche and first pregnancy), spacing out the intervals between births, or stopping child-bearing at a certain age. Such control does not assume the use of artificial means of fertility regulation or modern contraceptive methods but can result from the use of traditional means of contraception or pregnancy prevention (e.g., coitus interruptus), or from social norms or practices regarding celibacy, the age at marriage and the timing and frequency of sexual intercourse, including periods of prescribed sexual abstinence. Ansley Coale and other demographers have developed several methods for measuring the extent of such fertility control, in which the idea of a natural level of fertility is an essential component.
Proximate determinants describe variables that affect a female's fertility. There are seven proximate determinants of natural fertility, four of which affect the inter-birth interval:
- Age at menarche, which is the age at which a female starts her menstrual cycle
- Age at marriage, used to mark the period of time in which a female is sexually mature
- Inter-birth interval:
- Waiting time to conception, or the time it takes for the woman to become pregnant, including the time for sperm to travel to the egg and form a zygote
- Time added by fetal loss, also called postpartum infecundability, which is the amount of time necessary after a fetal loss for the womb to recover and be able to become fertile again
- Length of gestation, the nine-month period of fetal development in the womb
- Duration of lactational infecundability, which refers to the interval of time in which a mother is breastfeeding and usually cannot become pregnant
- Age at menopause, which is the age at which a female no longer has her menstrual cycle
Practicing natural fertility
- Hutterite communities in Russia, North and South Dakota, and Montana
- Old World Amish communities are prohibited from using any form of birth control by their religion and tend to have high fecundity rates
- !Kung San of Namibia, Botswana, and Angola do not practice any form of birth control. However, their total fertility rates are typically lower than other natural fertility populations due to low resources and therefore increased lactational infecundability. Infanticides may occur during these periods to compensate for overpopulation and to preserve resources.
Reasons for practice
Common reasons communities or individuals will practice natural fertility include concerns about developing medical conditions (including future infertility), pre-existing health conditions (including PCOS), cost of birth control, religious prohibition, lack of availability of birth control, and lack of information about birth control methods. Location also tends to be a factor in regards to the availability of both contraceptives and education on sexual practice. For example, less developed areas, including, but not limited to those extending throughout inland Africa lack access to the drugs necessary to control fertility or informative lessons describing their correct usage.
Influences on natural fertility rates
The number of children born to one woman can vary dependent on her window from menarche to menopause. Taking into consideration lactational infecundability and the period between conception and birth, the average woman is capable of experiencing around 20 births. However, if the duration of lactation is cut short due to use of a formula substitute or the woman has multiple births, the number of offspring could exceed 20.
Natural Fertility is not only influenced by women and their conscious decisions, but also their male counterparts. Even if a woman is unexposed to contraceptives, lacks knowledge of family planning, or purposely refrains from practicing regulated fertility, she could still struggle to conceive. Over the past half century, there has been an increase in scientific data supporting the decline in male sperm count. The decrease is attributed to various environmental toxins that are accumulating as the planet continues to industrialize. As long as sperm count remains above 60 million per ejaculate, fertility remains normal. Although, more and more males are approaching the 60 million boundary in correlation with the increase of atmospheric toxins. At such low levels, the sperm often are incapable of successfully fertilizing the egg. In result, women tend to run into more difficulties when trying to conceive, even if they are attempting to do so naturally.
[Preconditions for Fertility Decline]
Ansley J. Coale developed a theory to predict when a populations' fertility would begin to decline. His theory focused on three specific aspects. First, a couple must make conscious choice to control their fertility. This is closely related to secularization as some religions prohibit means of contraception. Second, there must be a benefit to controlling fertility that results in the desire for a smaller family. For example, as more regions move away from agriculture children are no longer needed to help with labor and fertility rates and family size tend to decrease. Third, the couple must be able to control fertility. This means that access to contraceptives or other means of limiting fertility must be available.
- Watcher, Kenneth W. (2013). Essential Demographic Methods. Harvard University Press.
- Wood, James W. (December 31, 1994). Dynamics of Human Reproduction: Biology, Biometry, Demography. Piscataway, New Jersey: Aldine Transaction. ISBN 978-0-202-01180-6.
- "Population explosion". The Economist. ISSN 0013-0613. Retrieved 2016-03-04.
- Martin, Robert (May 21, 2013). "Sperm Count Updated". Psychology Today. Sussex Publishers, LLC. Retrieved March 3, 2016.
- Coale, Ansley (1986). The Decline of Fertility in Europe. Princeton University Press. ISBN 0691094160.
- Nägele, F.K. (1843) Lehrbuch der Geburtshülfe. Erster Theil: Physiologie und Diätetik der Geburt. Mainz: Theodor von Zabern.
- Gibson, J.R. & McKeown, T. (1950) Observations on all births (23,970) in Birmingham, 1947. I: Duration of gestation. Brit. J. Soc. Med. 4:221-233.
- McKeown, T. & Record, R.G. (1952) Observations on foetal growth in multiple pregnancy in man. J. Endocrinol. 8:386-401.
- Henry, Louis (1961). "Some data on natural fertility", Eugenics Quarterly 8: 81–91.
- Coale, Ansley J. (1971). "Age Patterns of Marriage", Population Studies 25: 193-214.
- Coale, Ansley J., and James T. Trussell (1974). "Model fertility schedules: Variations in the age structure of childbearing in human populations", Population Index 40: 185–258.
- ——— (1975). "A new method of estimating standard fertility measures from incomplete data", Population Index 41: 182–210.
- ——— (1978). "Finding the two parameters that specify a model schedule of marital fertility rates", Population Index 44: 203–13.
- Leakey, M. D. 1984. Disclosing the Past. London: Weidenfeld & Nicolson.
- Wilson, Chris, Jim Oeppen, and Mike Pardoe (1988). "What is natural fertility? The modeling of a concept", Population Index 54 (1): 4–20.
- Xie, Yu. (1990). "What is Natural Fertility? The Remodeling of a Concept", Population Index 56 (4): 656-663.
- Bergsjø, P., Denman, D.W., Hoffman, H.J. & Meirik, O. (1990) Duration of human singleton pregnancy. A population-based study. Acta Obstet. Gynecol. Scand. 69:197-207.
- Wood JW (1994) Dynamics of Human Reproduction: Biology, Biometry, Demography. Hawthorne, N.Y.: Aldine de Gruyter Publishers.
- Ember, Carol R., and Martin Ember (2004). Encyclopedia of Medical Anthropology: Health and Illness in the World's Cultures. Vol. 2. New York: Springer.
- Jukic, A.M., Baird, D.D., Weinberg, C.R., McConnaughey, D.R. & Wilcox, A.J. (2013) Length of human pregnancy and contributors to its natural variation. Hum. Reprod. 28: 2848-2855.
- Watcher, Kenneth W. (2013). Essential Demographic Methods. Harvard University Press.
- Bongaarts, John. "A Framework for Analyzing the Proximate Determinants of Fertility." Pop Council. Web. 3 Mar. 2016.
- [Weeks] Weeks, John R. Population: An Introduction to Concepts and Issues, 12th Belmont, CA: Wadsworth/Thomson Learning.