Analysis guide
Population Growth vs. Fertility Rate
Learn why fertility and population growth can move in different directions, and how mortality, migration, and age structure connect them.
Fertility rate and population growth are connected, but they are not the same measure and do not always move together.
The total fertility rate describes births per woman under the age-specific rates of a period. Population growth measures the change in the number of people. To move from one to the other, you must also account for the number of potential parents, survival, deaths, and migration.
The population balance
At its simplest:
Population change = births − deaths + immigration − emigration
Fertility affects births, but not by itself. The number and age distribution of women in reproductive ages matters too. Mortality determines deaths and survival. Migration can add or remove people at any age, though international migration is often concentrated among younger adults.
Falling fertility with continued growth
This is common during demographic transition. Fertility may fall from a high level while the population continues to grow because:
- a large young generation is entering reproductive ages;
- death rates remain low or are falling;
- net migration is positive;
- births still exceed deaths even at the lower fertility rate.
The growth rate can slow for years before becoming zero. A narrowing pyramid base is therefore compatible with a rising total population.
Low fertility without decline
A population with below-replacement fertility can remain stable or grow if net migration offsets natural decrease. It can also grow through momentum when large adult cohorts produce many births in total.
This is why “fertility below 2.1” and “shrinking population” should never be treated as synonyms.
Rising fertility with continued decline
After long periods of low fertility, the reproductive-age population may be small relative to older groups. An increase in births per woman may not immediately produce enough births to exceed deaths. Negative momentum can persist.
The number of births depends on both the rate and the population exposed to it.
Rate versus absolute change
A small country can have a very high percentage growth rate while adding relatively few people. A large country can add tens of millions with a much lower rate. Our ranking cluster separates fastest growth rates from largest numerical increases by 2050.
Neither is the universally “correct” ranking. The useful measure depends on the question:
| Question | Better measure |
|---|---|
| How quickly is the population changing relative to its size? | Annual growth rate |
| How many additional people may need services? | Absolute population change |
| Is change driven by births and deaths? | Natural increase |
| What is migration’s contribution? | Net migration |
How to investigate a country
Start with its population pyramid, not a headline rate. Compare 2023, 2026, and 2050; check whether younger cohorts are getting smaller; note the size of reproductive-age groups; then read fertility, life expectancy, growth, and migration as separate evidence.
The country comparison tool places several of those measures together. The projection guide explains why future values remain conditional.
Sources
Frequently asked questions
Can population grow while fertility falls?
Yes. Large cohorts entering reproductive ages, longer survival, and net immigration can sustain growth while births per woman decline.
Can fertility rise while population declines?
Yes. If the population has relatively few people in reproductive ages and many older people, deaths can still exceed births despite a higher fertility rate.
What is natural population growth?
Natural increase is births minus deaths. Total population change also includes net international migration.