Human Fertility
Here's an abbreviated wiki article on
the subject. The entire article (with
references) can be found at: http://en.wikipedia.org/wiki/Fertility
Bottom
line
Birth control has been practiced by humans for
thousands of years. You figure it out, like what you want to do.
One story. During one deployment in Korea,
our battalion made anyone going into town carry two condoms. Our Catholic
chaplain was furious, but the rules are the rules. Of course he could have just
stayed back in the cantonment, but the town offered a warm bath, cooked warm
food, cold beer, and warm sex. Prostitution was legal. Well, our VD/STD rate turned
out to be lower than most USA urban areas at the time (circa the mid 1980's).
Fertility
From Wikipedia, the free encyclopedia
Fertility is the natural human capability of producing offspring. 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.
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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
- 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.[4]
- 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:[5]
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),[6]
Mincer (1963),[7]
and Easterlin (1969).[8]
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.
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.
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 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.[9]
This fertile window varies from woman to woman, just as the ovulation date
often varies from cycle to cycle for the same woman.[10]
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.[11]
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.[12][13]
Women's fertility peaks in the early
20s, and drops considerably after age 35.[14]
Menopause
typically occurs during a women's midlife (usually between ages 45 and 55 [15][16]).
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 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[17]
- 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.[18]
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.[19][20]
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".[21]
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."[22]
Male
fertility
Evidence shows that increased male
age is associated with a decline in semen volume, sperm motility,
and sperm morphology.[23]
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%.[23]
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.[24]
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.[25]
Increased paternal age has also been correlated to schizophrenia in numerous
studies.[26][27][28]
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.[29]
The American
Fertility Society recommends an age limit for sperm
donors of 50 years or less,[30]
and many fertility clinics in the United Kingdom
will not accept donations from men over 40 or 45 years of age.[31]
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.[32]
Historical
trends by country
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.[33]
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.
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.[35]
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
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.[36]
Ovulation Calculator
Here's one link you can try:
http://www.webmd.com/baby/healthtool-ovulation-calculator
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