Population momentum is a loose term referring to the rate at which the population would grow if the current generation of childbearing women started producing children at replacement fertility, i.e., they are having enough children to replace themselves.
The concept is fuzzy because replacement fertility refers to lifetime completed fertility and does not specify the exact age at which the female has children, rather than to the crude birth rate, and it is the latter, in conjunction with the crude death rate and net migration rate, that would determine the extent of population growth. However, loosely speaking, whether the population momentum would be positive (i.e., we would have population growth at replacement fertility) depends on how the size of the current childbearing generation compares with the size of the oldest generation that is dying out. The larger the former is relative to the latter, the higher the population momentum. This means that high fertility rates in a previous generation, that result in a high ratio of people in childbearing years to old people (aka a young population), would result in a highly positive population momentum.
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