The trend will lead to a "baby boom" and "baby bust" divide across the world, with the boom concentrated in low-income countries that are more susceptible to economic and political instability, senior researcher Stein Emil Vollset from the Institute for Health Metrics and Evaluation (IHME) at the University of Washington in Seattle said in a statement.
The study reported in The Lancet projects 155 of 204 countries and territories worldwide, or 76%, will have fertility rates below population replacement levels by 2050. By 2100, that is expected to rise to 198, or 97%, researchers estimated.
The forecasts are based on surveys, censuses, and other sources of data collected from 1950 through 2021 as part of the Global Burden of Diseases, Injuries, and Risk Factors Study.
Over three-quarters of live births will occur in low- and lower-middle-income countries by the end of the century, with more than half taking place in sub-Saharan Africa, researchers said.
The global fertility rate - the average number of births per woman - has fallen from around 5 children in 1950 to 2.2 in 2021, data show.
By 2021, 110 countries and territories (54%) had rates below the population replacement level of 2.1 children per woman.
The study highlights a particularly worrying trend for countries like South Korea and Serbia, where the fertility rate is less than 1.1 child per female, exposing them to challenges of a dwindling workforce.
Many of the most resource-limited countries "will be grappling with how to support the youngest, fastest-growing population on the planet in some of the most politically and economically unstable, heat-stressed, and health system-strained places on earth,” Vollset said.
While tumbling fertility rates in high-income countries reflect more opportunities for education and employment for women, researchers said the trend signals an urgent need for improvement in access to modern contraception and female education in other regions.
In addition, "once nearly every country's population is shrinking, reliance on open immigration will become necessary to sustain economic growth," IHME's Natalia Bhattacharjee, a coauthor of the report, said in a statement.
The authors noted that predictions were limited by quantity and quality of past data, especially for the 2020 to 2021 COVID-19 pandemic period.
