The study, published on medical journal The Lancet on Tuesday, forecasts population scenarios from 2018 to 2100 for 195 countries and territories using estimates from the 2017 Global Burden of Disease Study (GBD), an observational epidemiological study that describes mortality and morbidity from major diseases, injuries and risk factors to health at global, national and regional levels.

Vietnam would reach its peak population the fifth quickest among 13 countries and territories in Southeast Asia, after Thailand (71.97 million) in 2028, Mauritius (1.3 million) and Sri Lanka (22.34 million) in 2031, and Seychelles (110,000) in 2042.

The region is expected to reach its peak population of 786.84 million in 2052.

The U.N. last year forecast Vietnam’s population to peak at 109.795 million in 2054.

The Lancet study said by 2100 Vietnam’s population would shrink to 72.85 million, a 24.2 percent reduction from 2017 (96.14 million), the fourth highest reduction in the region behind Sri Lanka’s 51.6 percent (21.6 million to 10.45 million), Thailand’s 50.9 percent (70.63 million to 34.66 million) and Mauritius’s 43.3 percent (1.27 million to 0.72 million).

Only four countries in the region, Malaysia, the Maldives, the Philippines, and Timor Leste, would see their populations increase by 2100, it said.

The U.N. last year said Vietnam’s population would be 97.437 million by 2100.

The Lancet study also predicted the world population to peak at 9.732 billion in 2064, before falling to 8.785 billion by 2100 amid declining fertility rates and graying populations.

Out of 195 countries and territories in the study, 183 would have fallen below the replacement threshold needed to maintain population levels by 2100.

"These forecasts suggest good news for the environment, with less stress on food production systems and lower carbon emissions, as well as significant economic opportunity for parts of sub-Saharan Africa," AFP quoted Christopher Murray, director of the Institute for Health Metrics and Evaluation at the University of Washington and lead author of the study, as saying.

"However, most countries outside of Africa will see shrinking workforces and inverting population pyramids, which will have profound negative consequences for the economy."

Vietnam reached a turning point in 2015 when it started to become one of the countries with the fastest aging populations in the world, the Ministry of Labor, Invalids and Social Affairs said in a 2016 report.

Several localities have seen birth rates fall way below the ideal replacement fertility rate of two children per woman. For instance, in Ho Chi Minh City women are having around 1.36 children on average, while the rates in the southern provinces of Dong Thap, Hau Giang and Ba Ria-Vung Tau are 1.34, 1.53 and 1.37.

Low fertility rates would result in a quickly aging population, straining social welfare systems, including pensions, health insurance and social security, experts have warned.

The government has urged people to marry before 30 and bear children early, with women having their second child before 35.