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.

Assume that a population has three generations: First (oldest), Second (child bearing), and Third (children). Further assume that this population has a fertility rate equal to four (4). That is, each generation is twice the size of the previous. If the population of the first generation is arbitrarily set at 100, the second is then 200, and the third is 400. The spreadsheet below shows the initial population in the first row.

First note that the second and third generation of the initial population are each twice the size of the previous. The total of the initial population is 700 = 100 + 200 + 400.

Then assume that at the end of the third generation, fertility falls to replacement (for simplicity assume that to be two). Now take the population forward in time to the next generation, line two of the spreadsheet. The first generation dies, and the new generation, the fourth, is equal to the third (because now fertility is replacement). Repeat the process again to reach the fifth generation (line 3 in the spreadsheet). The fifth generation is again equal to the fourth and now the population’s three generations are equal, and the population has reached equilibrium.

The initial population has grown from 700 to 1,200 even though fertility dropped from four to replacement (two) at the end of the third generation. Population momentum carried the population to higher levels over the next two generations.

When China first introduced the one-child policy, population growth continued regardless. Even though the number of children born reduced dramatically, the sheer number of maturing youth was significant. In 1979 when the one-child policy entered into force, the number of people becoming adults was based on the number of births around the 1950s, not 1979. As a result, the Chinese population maintained the same momentum of increase as for the past 20 years. China's population shrunk for the first time only in 2023.

Population momentum impacts the immediate birth and death rates in the population that determine the natural rate of growth. However, for a population to have an absolute zero amount of natural growth, three things must occur.