Demographic momentum, also called population momentum, is the effect of a population’s age structure on its future growth or decline. A large generation reaching adulthood can sustain population growth even after fertility falls. Understanding this helps societies plan services; it does not justify directing anyone’s reproductive choices.

Why can population keep growing when fertility falls?

Fertility and population growth measure different things. The total fertility rate summarises births per woman under the age-specific fertility rates of a particular period. Total population change depends on births, deaths and migration. A fall in fertility therefore does not necessarily mean an immediate fall in population.

When a large generation reaches the ages at which people have children, there can be many births even if the average number of children per woman is lower than before. If births still exceed deaths, there is a natural increase. Migration can add to or offset that change within a country.

The UN’s World Population Prospects provides estimates and projections for these components. Population momentum is one part of the explanation, not a substitute for examining mortality, migration and changes in fertility.

A simple example of demographic momentum

Imagine two groups of women observed over their completed reproductive lives. In one group, 100 women have an average of three children each: 300 births. In a larger group, 200 women have an average of two children each: 400 births. The average falls, but the total number of births rises because the group is larger.

This is an illustrative calculation, not a country estimate or a population projection. It demonstrates the effect of group size. It does not calculate population growth: for that, we would also need deaths, migration and the timing of births. The lifetime averages here are not a calculation of a period total fertility rate.

Does replacement-level fertility stop growth immediately?

No. Replacement-level fertility concerns the longer-term replacement of generations; it does not instantly change the existing age structure. A youthful population may continue growing for a time after fertility reaches or falls below replacement. The often-used figure of about 2.1 births per woman assumes low mortality; it is not a universal threshold or a policy target.

For a fuller explanation of positive and negative momentum, see the US Census Bureau’s Population Momentum and Fertility technical note.

A dated example: India

UN DESA’s April 2023 policy brief reported India’s fertility rate at 2.0 births per woman in 2022, while projecting continued population growth for several decades. Its youthful age structure helps explain why the two trends can coexist. The same brief projected India’s population would overtake mainland China’s in April 2023, while noting uncertainty in the timing.

This is a historical example from the UN DESA policy brief published on 24 April 2023, not a current annual growth count.

What do UN projections say about momentum?

UN DESA’s World Population Prospects 2024 projects that population momentum embedded in the global age structure in 2024 will account for 79% of the increase in the world’s population through 2054, around 1.4 billion people. This is a global projection; age structures and population trajectories vary by country.

Population momentum helps explain one part of future change. Our explainer compares how UN, IHME and IIASA projections differ in release dates, assumptions and scenarios.

Planning for demographic change while protecting rights

There is no policy goal in demographic momentum itself. Reproductive rights and bodily autonomy matter in their own right. People should be able to make informed choices with access to voluntary family planning, education and healthcare; these rights and services do not depend on whether population growth rises or falls.

Family Planning: Voluntary family planning services and access to contraception support people to make informed choices about if and when to have children.

Education: Education is a right. Women and girls deserve equal access to education and information to make decisions about their lives.

Healthcare: Everyone should be able to access quality healthcare, including maternal and child health services, without access being conditioned on fertility or demographic outcomes.

Bodily autonomy: Women and girls have the right to make decisions about their bodies and reproductive lives, free from coercion.

Global justice: Ending poverty and advancing fair access matter in their own right, not as tools for changing birth rates.

The Future Outlook 

Planning should support people’s rights and wellbeing across generations, including in ageing societies.

Age-structure evidence can help plan schools, healthcare, housing and care services. Needs differ across generations and places, so national totals should be read alongside local evidence and people’s priorities.

Originally published 2 November 2023; explanation and sources reviewed 20 October 2026. Projections retain their release dates and are not guaranteed outcomes.