Fertility

Fertility is the natural capability to give life. In humans and animals, fertility means that the parent can produce babies.
More precisely, fertility is the capacity of an individual or population to produce viable offspring. By 'viable' is meant 'able to live and reproduce'.
In agriculture, fertility means that a soil can support the growth of plants because it contains the right minerals and nutrients.
Fertility can also be used as a metaphor: having a "fertile imagination" means being able to think of many new and interesting things.
As a measure, fertility rate is the number of children born per couple, or person or population. This is different to fecundity. This is defined as the potential for reproduction (influenced by gamete production, fertilisation and carrying a pregnancy to term. In 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, and may display astounding mechanisms.
Human fertility
Female Fertility
In the United States, the average age of menarche (first period) is about 12.5 years. However, in most girls, menarche does not signal that ovulation has occurred. 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 between ages 22-26, and after 35 it starts to sharply decline.
Birth rates have been noted by Henri Leridon, PhD, as 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 in vitro fertilization can increase the chances of becoming pregnant at a later age.
Male Fertility
There is evidence that male reproduction abilities are reduced with higher age.
Sperm count is becoming smaller with age, with men aged 50-80 years producing 75% of sperm compared with 20-50 years old males.
If the sperm count is too low, a man could increase it with herbal and chemical supplements.
Even larger change can be seen in the number of seminiferous tubules produced by males at different age stages:
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.
Infertility
When a person is infertile it means that they are unable to have a child naturally. Infertility is define as the inability to fall pregnant or carry a pregnancy until birth after 12 months or more of unprotected sex. About 10% of people are infertile. This means that 1 in 10 people will not be able to conceive a child naturally. There are many causes of infertility some of them can be treated. As a woman gets older, her fertility declines. After menopause a woman is no longer fertile.
References
Further reading
Bock, J. (2002). Evolutionary Theory and the search for a unified theory of fertility. American Journal of Human Biology. 14(2)'' 145-148. Full text
Reproductive system
Basic English 850 words