Human fertility depends on factors of nutrition, Human sexual activity|sexual behavior, culture, instinct, endocrinology, timing, economics, way of life, and emotions.
DemographyIn 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 (statistics)|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.*
Social determinants of fertilityThe "Three-step Analysis" of the fertility process was introduced by Kingsley Davis and Judith Blake in 1956 and makes use of three proximate determinants:*
Economic analysis of fertilityThe 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),* Mincer (1963),* and Richard Easterlin|Easterlin (1969).* The latter developed the Easterlin hypothesis to account for the Baby Boom.
Bongaarts' model of components of fertilityBongaarts 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. ;fertilisation|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 infection|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 Sportsperson|athletes and people susceptible to Anorexia nervosa|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 Infertility treatment|deal with subfecundity in women and men. An equally large industry has emerged to provide contraception|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 assumesthe 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 (medicine)|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 fertilityMen and women have Hormones|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 spermatozoon|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 cycleAlthough 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 fertilityThe average age of menarche in the United States is about 12.5 years.* 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.*Women's fertility peaks in the early 20s, and drops considerably after age 35.* Menopause typically occurs during a women's midlife (usually between ages 45 and 55 *). 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 *
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,* while another in 2013 of 2,820 Danish women saw 78% of 35-to-40-year-olds conceive within a year.*The use of fertility medication|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 Pregnancy over age 50#Cases of pregnancy over 50|women as old as 67.* 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.*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".* 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."*
Male fertilityEvidence 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;