U.N. demographers have anticipated since last year that the world's population may stop growing by 2100 as fertility rates decline, projecting a peak of 10.9 billion people by century's end, compared with roughly 7.8 billion now.

But a study published Tuesday in the Lancet, the medical journal, has challenged that forecast, with major economic and political implications. The study asserted that the global population could peak at 9.7 billion by 2064 — nearly four decades earlier — and decline to 8.8 billion by 2100.

Moreover, the study concluded, the elderly will make up a bigger chunk of the total than foreseen in the U.N. forecast, and the populations of at least 23 countries, including Japan, Thailand, Italy and Spain, could shrink by more than 50%. The study also projected significant declines in the working-age populations of China and India, the two most populous countries, portending a weakening in their global economic power.

The study's projections, if borne out, also carry significant consequences for the U.S., whose economy is expected to trail China's in size by 2035. As China's working-age population declines in the second half of the century, the study said, the U.S. could reclaim the top spot economically by 2098 — if immigration continues to replenish the U.S. workforce.

"Continued global population growth through the century is no longer the most likely trajectory for the world's population," said Dr. Christopher Murray, director of the Institute for Health Metrics and Evaluation at the University of Washington's School of Medicine, who led the study.

Murray said the study "provides governments of all countries an opportunity to start rethinking their policies on migration, workforces and economic development to address the challenges presented by demographic change."

An important underlying reason behind the conclusions is the improvement in access to modern contraception and the education of girls and women, which the study said would "hasten declines in fertility and slow population growth."

While the most recent U.N. population forecast, made in June 2019, also noted declining fertility, the new study said the consequences would be felt much sooner and with greater impact.

John Wilmoth, director of the population division in the U.N. Department of Economic and Social Affairs, which produces the organization's projections every two years, said that it had made some assumptions about fertility, mortality and migration that helped shape the conclusions. One of the biggest assumptions, he said, was that countries with low fertility rates would do nothing to raise them between now and 2100. "It's kind of an extreme assumption to think that countries aren't going to think their way out of the problem for the next 80 years," he said.

Wilmoth also said the United Nations had been tracking population trends for 70 years and that its projections "represent a consensus view" among demographers. Still, he said, "I welcome this sort of creative inquiry about other ways of seeing these things."

The forecasting methodology used in the study found that by 2100, 183 of 195 countries would have total fertility rates — the average number of children a woman delivers over her lifetime — below the replacement level of 2.1 births. That is the level needed to prevent population decline.