fertility

fertility, fertility rate

The actual level of childbearing of an individual or population. There are different ways of measuring the fertility of a population. The simplest measure, the crude birth-rate , relates the number of live births in a given year to the total population size in that year. More complex measures relate the year's births to more restricted populations, usually childbearing women, to yield a better index of underlying fertility. The denominator may be all women of childbearing years (commonly set at 15 to 44) or women in more specific age-bands (age-specific fertility rates). Fertility rates may be combined with mortality rates to generate an overall reproduction rate.

Measures relating fertility to population (whether total or specific) in a given year are termed period rates. Arguably of more value in detecting fertility trends are so-called cohort rates . These measure the births to women grouped according to year of birth (birth cohorts) or age at marriage (marriage cohorts). Cohort fertility rates can reveal variations in the timing of childbearing between different cohorts and indicate whether changes in period measures are due to changes in the spacing of births or to changes in overall family size (though final figures can only be obtained after a lengthy period).

Historically there has been a long-term decline in levels of fertility in industrial societies, a decline that is associated with economic development and restructuring, reductions in child mortality, changes in welfare provision and the economic and social value of children, and the changing social position of women.