A total fertility rate of 2.1 children per woman is the number typically cited as the replacement level, the point at which a population exactly replaces itself from one generation to the next. This figure applies to countries with low mortality rates. In places where fewer children survive to adulthood, the replacement rate can be significantly higher.
Why 2.1 and Not Exactly 2.0
Two parents need two children to replace themselves, so you might expect the magic number to be 2.0. The extra 0.1 accounts for two biological realities. First, slightly more boys are born than girls, roughly 105 boys for every 100 girls. Since only women bear children, a population needs slightly more total births to produce enough future mothers. Second, not every child survives to reproductive age. Even in wealthy countries with excellent healthcare, a small fraction of children die before they can have children of their own. That combination of a skewed sex ratio and modest childhood mortality pushes the number from a neat 2.0 to approximately 2.1.
The Rate Varies by Country
The 2.1 figure assumes low mortality, which makes it a reliable benchmark for high-income countries but not a universal constant. In nations where child survival rates are much lower, far more births are needed to ensure enough children reach adulthood and reproduce. Research has found that in countries where only about 60% of children survive to reproductive age, such as Afghanistan, Burundi, and Sierra Leone, the replacement fertility rate climbs to around 3.3 children per woman.
Sub-Saharan Africa illustrates the gap clearly. The region’s average fertility rate is about four children per woman, nearly double the global average. While that’s well above 2.1, it partly reflects the higher replacement threshold created by child mortality, alongside limited access to contraception and lower rates of female education. Projections suggest that achieving universal female education or meeting unmet needs for modern contraception in those countries by 2030 could bring fertility rates down to around 2.3 by 2050, much closer to long-term stability.
Where the World Stands Now
The global fertility rate in 2024 was 2.2 births per woman, a dramatic decline from around 5 in the 1960s and 3.3 as recently as 1990. That global average, however, masks enormous variation. Over half of all countries, representing about 68% of the world’s population, already have fertility rates below 2.1. These aren’t just wealthy Western nations. They span every region and income group.
Population scientists estimate there is a 90% probability that global fertility will be below replacement level by 2250 or sooner, and nearly all central projections agree that fertility will stay below replacement for decades at minimum. The question is no longer whether most countries will fall below 2.1, but what happens after they do.
Why Population Growth Continues After Fertility Drops
One of the most counterintuitive facts in demography is that a population can keep growing for decades after its fertility rate hits replacement level. This is called population momentum, and it happens because of the age structure left behind by earlier generations of high fertility.
When a country has had many years of large families, it ends up with a disproportionately large number of young women entering their childbearing years. Even if each of those women has only two children, the sheer number of mothers produces more births than deaths for a generation or two. The demographer Nathan Keyfitz demonstrated this effect in the 1970s, showing that an immediate, overnight drop to replacement fertility in a high-fertility population would still result in decades of continued growth. For countries that haven’t yet completed this demographic shift, momentum will be a major driver of population increase as large cohorts of children grow up and start families.
What Happens Below Replacement
Sustained fertility below 2.1 eventually leads to population aging and, if it persists long enough, population decline. The economic consequences center on the balance between workers and dependents. When fertility is low and life expectancy is long, a growing share of the population is retired, consuming public resources like pensions and healthcare while no longer contributing labor income. Japan offers a concrete example: its support ratio, the balance of workers to consumers, fell at 0.6% per year over the past decade, and its fiscal support ratio dropped even faster at 0.9% annually.
For high-income countries where fertility has dropped below 1.6, projections show that the stable ratio of workers to consumers would settle at about 80% of its 2010 level. That means fewer workers supporting more retirees, creating pressure on public programs and likely requiring adjustments to retirement ages or benefit structures. There is a partial offset: countries with fewer children tend to invest more heavily in each child’s education and skills, boosting the productivity of each worker. But this human capital dividend doesn’t fully compensate for a shrinking labor force.
How Migration Fills the Gap
Once a country reaches the demographic stage where births and deaths are roughly balanced, immigration becomes the primary engine of population growth. This dynamic is already playing out differently across the developed world. North America continues to grow in population largely because of immigration. In the United States, immigrants are directly and indirectly responsible for about two-thirds of total population growth. Most of Western Europe, which has historically been more restrictive on immigration, has seen some countries begin to lose population.
Migration doesn’t change the fertility rate itself, but it changes the population equation by adding people of working and childbearing age. For countries well below replacement fertility, immigration policy is effectively population policy, determining whether the total number of residents grows, stabilizes, or shrinks in the coming decades.