The fertility rate is a measure of the number of children that would be born to a woman if she were to live to the end of her childbearing years and bear children in accordance with age-specific fertility rates of the specified year. It is calculated by summing the age-specific fertility rates (ASFRs) over five-year intervals. The total fertility rate (TFR) is the average number of children that would be born to a female over her lifetime if she were to experience the exact current age-specific fertility rates through her lifetime and were to live from birth until the end of her reproductive life. The TFR is a commonly used measure of fertility and is directly calculated as the sum of ASFRs. Assuming no net migration and unchanged mortality, a TFR of 2.1 children per woman ensures a broadly stable population. The TFR varies widely across the world, from 0.78 in South Korea to 6.73 in Niger. The global TFR has declined rapidly since the 1960s, and as of 2020, the total fertility rate for the world is 2.3. The decline of the fertility rate is one of the most fundamental social changes that has happened in human history.