Bangladesh will be the 25th most populated country by 2100, with 81 million people, according to research by the University of Washington.
It will lose 48.2 percent of its population from 157 million in 2017. This decline is the highest in South Asia.
India will be the world's most populated country in 2100 with 1.09 billion people, the study said.
Currently, India has 1.3 billion people – second only to China (1.4bn). It is followed by: Nigeria (791m), China (732m), the US (336m), Pakistan (248m), and the Democratic Republic of Congo (246m).
All South Asian countries, except Pakistan, will face a decline in population by 2100. It will keep its fifth place in 2100 with a 16 percent increase from 214.29 million in 2017.
Bhutan will see its population fall by 19.8 percent from 0.96 million to 0.77 million.
Nepal will see the second-largest fall (39.5 percent) in population from 29.89 million to 18.09 million after Bangladesh.
The study was published on July 14 in the UK medical journal The Lancet.
Global population to peak in 44 years
The global population will peak in 44 years – at around 9.7 billion – before it starts to shrink again by 8.79 billion in 2100, the study said.
After 2064, the total global population will fall due to a decline in the number of children people are having.
The total fertility rate (TFR) of Bangladesh will drop to 1.19 which was 2.0 in 2017, but it is still 0.02 percentage points below the 2100 Sustainable Development Goals pace scenario.
On the other hand, the global total fertility rate was forecasted to be 1.66 in 2100 from 2.37 in 2017.
According to population experts, populations only grow or stay stable when women have an average of 2.1 children each – assuming no immigration or emigration occurs. Better education and contraception will stop this in the future because women will be able to work outside the home more.
According to the study, by 2050, 151 countries are forecasted to have a TFR lower than the replacement level (TFR<2·1), and 183 were forecasted to have a TFR lower than the replacement level by 2100.
Some countries, including Japan, Thailand and Spain, were forecasted to have a population decline of greater than 50 percent from 2017 to 2100.
Lead researcher of the study Dr Christopher Murray said, "Continued global population growth through the century is no longer the most likely trajectory for the world's population."
He also said that their work goes against the findings of the United Nations, which projects the population of the world will continue to grow for the rest of the century.
However, they acknowledged that the global population is likely to explode in the next 40 years, rising by more than two billion people.
Which economy will top the world in 2100?
The study also traced the economic consequences of population scenarios. The findings predicted that many of the world's top economies will remain powerful over the next 80 years, but there will be some reshuffling in the ranks.
China was forecasted to become the largest economy by 2035 but in the reference scenario, the US was forecasted to once again become the largest economy in 2098.
India will be the fourth largest economy in the world – in terms of total GDP by 2030 from its seventh position in 2017 – and will continue to remain in third position from 2050 to 2100.
By 2100, India was forecasted to still have the largest working-age population in the world, followed by: Nigeria, China, the US, and Pakistan.
Migration will be necessary
For many countries in other parts of Asia and in Europe, populations will shrink because birth rates are already low. In some countries, populations will drop by more than half.
In Japan, the researchers estimated the population will fall from around 128 million in 2017 to 60 million in 2100, a decline of 53 percent.
Some of the oldest countries will have to rely on immigration to make sure they have enough people to keep industries going, the scientists warned.
The research was done by a team of 24 scientists led by Dr Christopher Murray and Professor Stein Vollset from the University of Washington in Seattle.
The study forecasted population size from 2018 to 2100 in 195 countries. Mortality rate, fertility rate and migration were considered constant for each age, sex and location during a calendar year.