Answer: Population momentum is a consequence of the demographic transition. Population momentum explains why a population will continue to grow even if the fertility rate declines. Generally speaking, when the TFR is greater than 2.1, the population in a given area will increase, and when it is less than 2.1, the population in a given area will eventually decrease, though it may take some time because factors such as age structure, emigration, or immigration must be considered. Replacement level fertility will lead to zero population growth only if mortality rates remain constant and migration has no effect. The momentum of past and current demographic trends may also take several generations to work itself out.
America's fertility rate is in steady decline: In 2018, it dipped to an all-time low, down 2 percent from the year before, according to a Centers for Disease Control and Prevention report published Wednesday. American women are now predicted to have an average of 1.73 children over their lifetime.
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