Fertility rate - the average number of children women give birth to - is falling and health researchers believe the impact on the societies will be "jaw-dropping."
The global fertility rate was 2.4 in 2017, and is expected to fall below 1.7 by 2100, say researchers at the University of Washington's Institute for Health Metrics and Evaluation.
"That's a pretty big thing; most of the world is transitioning into natural population decline," researcher Prof Christopher Murray told the BBC.
"I think it's incredibly hard to think this through and recognise how big a thing this is; it's extraordinary, we'll have to reorganise societies."
According to a BBC report, 23 nations including Spain and Japan may see their populations halve by 2100.
Japan's population is expected to fall from 128 million in 2017 to less than 53 million by this century's end.
"That is jaw-dropping," Prof Christopher Murray told BBC.
Meanwhile, China is expected to peak at 1.4 billion in the next four years and fall to nearly 732 million in 2100. India is projected to take China's place.
The issue is said to be global with 183 out of 195 countries projected to have fertility rate below 1.7.
The greater access to contraception and birth control measures have led to women wanting fewer children compared to 1950s when women had 4.7 children at an average during their lifetime.
1950, women were having an average of 4.7 children in their lifetime.
Speaking about the problem that will be created in the society due to low fertility rate, Prof Murray said, "It will create enormous social change. It makes me worried because I have an eight-year-old daughter and I wonder what the world will be like."
