Population momentum or demographic inertia is the tendency of the raw birth rate to rise as a result of past high fertility rates, even after fertility rates have fallen, or vice-versa. This occurs because a current increase in fertility rates causes an increase in the number of women of childbearing age roughly twenty-to-forty years later, meaning population growth figures tend to lag substantially behind fertility rates. Well-known examples include the Echo Boom (the increase in the total number of births as baby boomers reached child-rearing age) and Chinese population growth throughout the era of the one-child policy (from 1979 until 2021).
Population momentum explains why a population will continue to grow even if the fertility rate declines or continues to decline even if the fertility rate grows. Population momentum occurs because it is not only the number of children per woman that determine population growth, but also the number of women of reproductive age. Eventually, when the fertility rate reaches the replacement rate and the population size of women in the reproductive age bracket stabilizes, the population achieves equilibrium and population momentum comes to an end. Population momentum is defined as the ratio of the size of the population at that new equilibrium level to the size of the initial population.
