Population momentum can be compared to a moving train — even when you apply the brakes (by lowering fertility rates), it continues to move forward for some time before coming to a complete stop.
Imagine a country where couples start having fewer children — below the replacement level of 2.1 children per woman. You might expect the population to stabilize or even decline immediately. However, in reality, it continues to grow for several decades. Why does this happen?
Even with fewer births per family, if a country already has a large number of young people (especially in their 20s and 30s), many of them will still have children. Although each family is smaller, the total number of births remains high because of the large base of potential parents.
India’s fertility rate has fallen below 2.1 in many states. Yet, the population continues to grow. This is due to the country’s large youth population — even if each couple has fewer children, the sheer number of couples keeps the overall population rising for years.
Think of a school that used to admit 100 students every year. Suddenly, it reduces new admissions to 50. Even though new students are fewer, the school remains crowded for a while because older students are still in other grades. Over time, as they graduate, the total student number slowly decreases.
Population momentum means that even when fertility falls below the replacement level, population growth does not stop immediately. It takes one or more generations for the full effects to appear. This is why countries planning for ageing populations or population decline must act early — demographic changes unfold gradually, not overnight.
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