Fertility

Fertility is the natural capability of giving life. As a measure, "fertility rate" is the number of children born per couple, person or population. Fertility differs from fecundity, which is defined as the potential for reproduction (influenced by gamete production, fertilisation and carrying a pregnancy to term). A lack of fecundity would be called sterility.

Human fertility depends on factors of nutrition, sexual behavior, 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. 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.

Period measures

 * Crude birth rate (CBR) - the number of live births in a given year per 1,000 people alive at the middle of that year. One disadvantage of this indicator is that it is influenced by the age structure of the population.
 * General fertility rate (GFR) - the number of births in a year divided by the number of women aged 15–44, times 1000. It focuses on the potential mothers only, and takes the age distribution into account.
 * Child-Woman Ratio (CWR) - the ratio of the number of children under 5 to the number of women 15-49, times 1000. It is especially useful in historical data as it does not require counting births. This measure is actually a hybrid, because it involves deaths as well as births. (That is, because of infant mortality some of the births are not included; and because of adult mortality, some of the women who gave birth are not counted either.)
 * Coale's Index of Fertility - a special device used in historical research

Cohort measures

 * Age-specific fertility rate (ASFR) - The number of births in a year to women in a 5-year age group, divided by the number of all women in that age group, times 1000. The usual age groups are 10-14, 15-19, 20-24, etc.
 * Total fertility rate (TFR) - the total number of children a woman would bear during her lifetime if she were to experience the prevailing age-specific fertility rates of women. TFR equals the sum for all age groups of 5 times each ASFR rate.
 * Gross Reproduction Rate (GRR) - the number of girl babies a synthetic cohort will have. It assumes that all of the baby girls will grow up and live to at least age 50.
 * Net Reproduction Rate (NRR) - the NRR starts with the GRR and adds the realistic assumption that some of the women will die before age 59; therefore they will not be alive to bear some of the potential babies that were counted in the GRR. NRR is always lower than GRR, but in countries where mortality is very low, almost all the baby girls grow up to be potential mothers, and the NRR is practically the same as GRR. In countries with high mortality, NRR can be as low as 70% of GRR. When NRR = 1.0, each generation of 1000 baby girls grows up and gives birth to exactly 1000 girls. When NRR is less than one, each generation is smaller than the previous one. When NRR is greater than 1 each generation is larger than the one before. NRR is a measure of the long-term future potential for growth, but it usually is different from the current population growth rate.

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:

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 use 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. "Involunatry fetal mortality" involves natural abortionmiscarriages and stillbirth (a fetus born dead). Human intervention intentionally causing abortion of the fetus is called "therapeutic abortion".

Human fertility
Both women and men 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 the male cycle is 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
Although women can become pregnant at any time during their menstrual cycle, peak fertility occurs during just a few days of the cycle: usually two days before and two days after 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 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
The average age of menarche in the United States is about 12.5 years. In postmenarchal girls, about 80% of the cycles were anovulatory in the first year after menarche, 50% in the third and 10% in the sixth year. Women's fertility peaks between the ages of 22 to 26. However, 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. Results of the study are published in the journal Science. With a rise in women postponing pregnancy, this can create an infertility problem.

According to the March of Dimes, "about 9 percent of recognised 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".

In 2010, researchers at the University of Edinburgh determined that by the time women are 30 years old, only 10% of their non-growing follicles (NGFs) remain. At birth, women have all their follicles for folliculogenesis, and they steadily decline until menopause.

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."

According to 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:
 * At age 30
 * 75% will have a conception ending in a live birth within one year
 * 91% will have a conception ending in a live birth within four years.
 * At age 35
 * 66% will have a conception ending in a live birth within one year
 * 84% will have a conception ending in a live birth within four years.
 * At age 40
 * 44% will have a conception ending in a live birth within one year
 * 64% will have a conception ending in a live birth within four years.

The use of fertility drugs and/or invitro fertilization can increase the chances of becoming pregnant at a later age. Successful pregnancies facilitated by fertility treatment have been documented in women as old as 67.

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

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;


 * In males 20–39 years old, 90% of the seminiferous tubules contain mature sperm.
 * In males 40–69 years old, 50% of the seminiferous tubules contain mature sperm.
 * In males 80 years old and older, 10% of the seminiferous tubules contain mature sperm.

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. Increased paternal age has also been correlated to schizophrenia in numerous studies.

Australian researchers have found evidence to suggest overweight obesity may cause subtle damage to sperm and prevent health pregnancy. They say fertilization was 40% less likely to succeed 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. 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.

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.

Post-WW II USA
From 1800 to 1940, fertility fell steadily in the US. Then suddenly it 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. After 1960, ideal family size fell sharply, from 3 to 2 children. Couples postponed marriage and first births, and they sharply reduced the number of third and fourth births.

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.