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fertility

[fer-til-i-tee]

fertility: see infertility.

The Columbia Electronic Encyclopedia Copyright © 2004.
Licensed from Columbia University Press

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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. This is different from fecundity, which is defined as the potential for reproduction (influenced by gamete production, fertilisation and carrying a pregnancy to term). In the English language, the term was originally applied only to females, but increasingly is applied to males as well, as common understanding of reproductive mechanisms increases and the importance of the male role is better known. Infertility is a deficient fertility.

Human fertility depends on factors of nutrition, sexual behavior, culture, instinct, endocrinology, timing, economics, way of life, and emotions. Animal fertility is no less complex.

Agriculture

Fertility is also applied to farmlands and plants, where it implies a capacity to yield large crops of sound fruits, seeds or vegetables.

Demography

The fertility rate is a demographic measure of the number of children per woman. Although it has been until recently considered to be a fairly reliable indicator of population growth, it is no longer so in much of Asia. Due to selective abortion and other factors, the ratio of women relative to men in populations is declining. Therefore, the fertility rate as it has traditionally been defined is no longer an authoritative measure of population growth in China, India,Myanmar, and Korea.

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 a role, especially for 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 between from woman to woman, just like the ovulation date often varies from cycle to cycle for the same woman. The ovule is usually fertilisable for up to 48 hours after it is released from the ovary. However, sperm can survive inside the uterus between 48 to 72 hours on average, with the maximum being 120 hours (5 days).

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 around the age of 19-24, and often declines after 30. With a rise in women postponing pregnancy, this can create an infertility problem. Of women trying to get pregnant, without using fertility drugs or in vitro fertilization:

At age 30, 75% will get pregnant within one year, and 91% within four years.
At age 35, 66% will get pregnant within one year, and 84% within four years.
At age 40, 44% will get pregnant within one year, and 64% within four years.

The above figures are for pregnancies ending in a live birth and take into account the increasing rates of miscarriage in the ageing population. 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.

The use of fertility drugs and/or in vitro 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.

Doctors recommend that women over 30 who have been unsuccessful in trying to conceive for more than 6 months undergo some kind of fertility testing.

Male fertility and age

Erectile dysfunction increases with age, but fertility does not decline in men as sharply as it does in women. There have been examples of males being fertile at 94 years old. However, evidence suggests 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%.

Cause of decline

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.

Recent research has suggested increased risks for health problems for children of older fathers. A large scale Isreali 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.

The American Fertility Society recommends an age limit for sperm donors of 50 years or less. Many fertility clinics in the United Kingdom will not accept donations from men over 40 or 45. Many women are using a take-home baby rate calculator to estimate their chances of success following in vitro fertilisation.