The level of childbearing; in an individual, but more often in a society or nation. Crude birth rate is the simplest measure of fertility, but does not relate the number of births to the number of women of childbearing age, while the general fertility rate, or fertility ratio, shows the number of births in a year per 1 000 women of reproductive age (generally 15–45, sometimes 15–49). Cohort fertility rates show the number of births to women grouped according to either their year of birth or their year of marriage. The total fertility rate is the average number of children that would be born per woman, if women experienced the age-specific fertility rates of the year in question throughout their childbearing lifespan. Globally, fertility rates vary widely; in 2006, the rate for the UK was 1.84; for Malawi, 5.7. Abernethy (1999) Pop. & Env. 21, 2 argues for the fertility opportunity hypothesis—that humans are alert to environmental signs that indicate whether conditions for childbearing and nurture are more or less optimal.

There are very strong global correlations between fertility rates and per capita GNP, fertility rates and women’s education, and fertility and infant mortality rates (Dasgupta in K.-G. Mäler and J. Vincent, eds 2001). Abernethy (2004) AAAS Annual Meeting proposes a link between energy and fertility. Fertility has been declining in industrial societies since the late nineteenth century; a decline which preceded easily available artificial contraception.

http://esa.un.org/wpp/Documentation/pdf/WPP2012_Volume-I_Comprehensive-Tables.pdf UN website with tables on estimated and projected total fertility for the world, development groups, and major areas (p. 11).