The total fertility rate (TFR, also called fertility rate or total period fertility rate (TPFR)) of a population is the average number of children that would be born to a woman over her lifetime if she were to experience the current age-specific fertility rates through her lifetime. It is obtained by summing the age-specific rates for a given time-point.

The TFR is a synthetic rate, not something that is actually counted. It is not based on the fertility of any real group of women, since this would involve waiting until they had completed childbearing. Nor is it based on counting up the total number of children actually born over the lifetime, but instead is based on the age-specific fertility rates of women in their "child-bearing years," which in conventional international statistical usage is ages 15-49.

The TFR is therefore a measure of the fertility of an imaginary woman who passes through her reproductive life and is subject to all the age-specific fertility rates for ages 15-49 that were recorded for a given population in a given year. The TFR represents the average number of children a woman would have were she to fast-forward through all her childbearing years, assuming the age-specific fertility rates for a given year. This rate, then, is the number of children a woman would have if she was subject to prevailing fertility rates at all ages, and survives throughout all her childbearing years.

A more accurate fertility measure is the Net Reproduction Rate (NRR) which measures the number of daughters a woman would have in her lifetime if she were subject to prevailing fertility rates in the given year, and also takes into account prevailing mortality rates in that year. However, this rate is less widely used, and the United Nations stopped reporting NRR data for member nations after 1998.

The TFR (or TPFR) is a better index of fertility than the Crude birth rate (annual number of births per thousand women of childbearing age) because it is independent of the age structure of the population, but it is a poorer estimate of actual completed family size than the total cohort fertility rate, which is obtained by summing the age-specific fertility rates that actually applied to each cohort as they aged through time. In particular, the TFR does not predict how many children young women now will eventually have, as their fertility rates in years to come may well differ from those of older women now. However, the TFR is a good summary of current fertility levels.

Replacement rates

Replacement fertility is the total fertility rate at which women would have only enough children to replace themselves and their partner. By definition, replacement is only considered to have occurred when the offspring reach 15 years of age. If all offspring survived to the age of 15 the replacement rate would be exactly 2, but in practice it is affected by childhood mortality. The replacement fertility rate is roughly 2.1 births per woman for most industrialized countries and has not been evaluated for poorer countries. At this rate, population growth through reproduction will be approximately zero, but will also be affected by male-female ratios and mortality rates.

Developed countries usually have a much lower fertility rate due to greater wealth and their individualistic culture. Mortality rates are low, birth control is easily accessible, and children often can become an economic drain caused by education costs, clothing and feeding. Longer periods of time spent getting higher education often mean young people have children later in life.

In developing countries on the other hand, children are a necessity as labour in fields and as caregivers and providers of their parents in old age. Fertility rates are also higher due to the lack of access to contraceptives.

The total fertility rate in the United States after World War II peaked at about 3.8 children per woman in the late 1950s and by 1999 was at 2 children. This means that a woman who was 15 years old in the late 1950s would have been expected to have about four children by the time she reached 44 years of age, whereas a 15-year-old woman in 1999 will be expected to have only about two children in her lifetime.

A population that maintains a TFR of 3.8 over a long time would increase rapidly, whereas a population that maintains a TFR of 2.0 over a long time would decline (unless it had a large enough immigration). The TFR required for a population to remain constant in size is 2.1 (assuming mortality rates are not unusually high). However, when a population achieves replacement-level fertility, that population will continue to grow for several generations, approximately 50 to 200 years. This phenomenon is called population momentum or population-lag effect. This time-lag effect is of great importance to human populations growth rates.

See also

• Crude birth rate
• List of countries and territories by fertility rate
• Demography

External links

• World Factbook table of Total Fertility Rate ordered by country name
• World Factbook table of Total Fertility Rate ordered by country rank
• Population Reference Bureau Glossary of Population Terms

Population

Plodnost | Fertilitätsrate | Tasa de fertilidad | Termékenységi ráta | Samlet fruktbarhetstall | Taxa de fecundidade | Коэффициент фертильности | 合計特殊出生率 | Taux de fécondité