THURSDAY, March 21, 2024 (HealthDay News) -- By 2050, three-quarters of the world's nations will see fertility rates fall to below replacement levels, meaning their populations will be steadily shrinking, a new study predicts.

And by 2100, almost all countries (97%) are expected to have fertility rates below the replacement level of 2.1 children per woman, the same report concludes.

This trend will not happen everywhere all at once. Richer countries will be hit first and hardest by falling birth rates, with poorer nations maintaining higher birth rates. That's according to researchers at the Institute for Health Metrics and Evaluation (IHME) at the University of Washington School of Medicine in Seattle.

All of this means big shifts in where the world's babies are being born.

According to the report, 29% of babies were born in sub-Saharan Africa in 2021. But by 2100, over half (54%) of all infants will be born in that region, should current trends persist.

The findings were published March 20 in The Lancet journal.

“We are facing staggering social change through the 21st century,” said study senior author Stein Emil Vollset, from IHME.

"The world will be simultaneously tackling a 'baby boom' in some countries and a 'baby bust' in others," he explained in a journal news release. "As most of the world contends with the serious challenges to the economic growth of a shrinking workforce and how to care for and pay for aging populations, many of the most resource-limited countries in sub-Saharan Africa 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.” 

As Vollset's team noted, much of the decline in fertility rates has already long been underway.

In 1950, the average woman globally gave birth to about five children; by 2021 that had shrunk to 2.2 (just over replacement level), with 110 of 204 countries already charting fertility rates below replacement level.

In some countries, such as South Korea or Serbia, fertility has fallen to levels as low as 1.1, according to the report.

In contrast, the fertility rate of the sub-Saharan nation of Chad stood at 7 in 2021.

There are a lot of positive reasons that many women -- especially those in more affluent countries -- are having fewer children.

“In many ways, tumbling fertility rates are a success story, reflecting not only better, easily available contraception but also many women choosing to delay or have fewer children, as well as more opportunities for education and employment," Vollset explained.

Those are changes that, if replicated in Africa and elsewhere, might help curb burgeoning populations in resource-poor nations.

“It’s clear that tackling the population explosion in higher-fertility countries depends greatly on accelerating progress in education for girls and reproductive rights,” said study  co-lead author and acting assistant professor from IHME, Dr. Austin Schumacher.

Meanwhile, some richer nations are already promoting "pro-natal" policies to boost their fertility rates. The Seattle researchers say those policies probably can't bring birth rates back up to replacement levels, but they might stop them from hitting serious lows.

“There’s no silver bullet,” said co-lead author and IHME lead research scientist Dr. Natalia Bhattacharjee. “Social policies to improve birth rates such as enhanced parental leave, free childcare, financial incentives and extra employment rights, may provide a small boost to fertility rates, but most countries will remain below replacement levels."

What's clear is that in coming decades, what the study authors describe as a 'demographically divided world’ will place pressure on rich and poor nations alike -- especially when it comes to emigration and immigration.

"Once nearly every country’s population is shrinking, reliance on open immigration will become necessary to sustain economic growth," Bhattacharjee said. "Sub-Saharan African countries have a vital resource that aging societies are losing -- a youthful population.” 

More information

Find out about U.S. fertility rates at the University of Wisconsin - Madison.

SOURCE: The Lancet, news release, March 20, 2024