Total Fertility Rate
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TFR is an abbreviation for Total Fertility Rate, which is the number of babies born to every woman during their reproductive years.
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 chilbearing. The TFR is therefore a measure of the fertility of an imaginary woman who passes through her reproductive life and is subject to all age-specific fertility rates that were actually recorded in a given year. Therefore, 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.
The total fertility rate in the United States 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 maintained a TFR of 3.8 over a long time would increase rapidly, whereas a population that maintained a TFR of 2.0 over a long time would decline. The TFR required for a population to remain constant in size is 2.1. From the day that a population achieves replacement-level fertility, however, 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. See also: DemographyTemplate:Socio-stub