Reproductive rights and bodily autonomy matter in their own right. Demographic momentum describes how a youthful age structure can sustain population growth after fertility falls. It is a concept for understanding and planning for population change, not a reason to set population targets or direct people’s reproductive choices.

Demographic momentum (also known as population momentum) is like the hidden engine that drives population growth. Even if a country’s birth rates and family sizes drop, its population can still keep growing due to the age structure of the population. This means that the impact of smaller family sizes can sometimes take many decades to be reflected in population numbers. 

Ages and stages

Every society obviously has people of all ages, from newborn babies to the very elderly. But the key players in demographic momentum are the young adults, those in their childbearing years. When there’s a large number of these young adults, even if they have fewer kids each on average (known as fertility rate), the sheer number of potential parents can mean more babies in total and ongoing population growth. 

This means that countries with fertility rates below “replacement level” often continue to experience natural increases for some time. This is most common where birth rates have recently been high – when the babies born in that period reach adulthood they increase the number of families.

Today, we can see this clearly in countries like India. The average fertility rate (the number of births per woman) has reduced significantly in recent decades, from 5.9 in 1950 to below 2 today (2.1 is generally considered the replacement rate and, all other factors being equal, would eventually lead to a steady population). However, India’s population is still growing by over 10 million each year due to having a large number of young people (and people living longer), and earlier this year it overtook China as the most populous country in the world.

Demographic momentum is even more powerful in Africa. About half of the population of Africa is currently under 18. In most African countries fertility rates are coming down, but the sheer number of people moving into their child-bearing years will ensure a large number of births for decades to come. The UN projects that the continent will be where half of all global population growth takes place up to 2050.

UN DESA’s World Population Prospects 2024 projects that population momentum embedded in today’s global age structure 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.

What to do about Demographic Momentum

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.

In a world where we face complex and critical challenges such as climate change, understanding this phenomenon is crucial. It’s not just a numbers game; it’s about shaping the future and making informed decisions for the well-being of everyone.