The world's population will be around nine billion by 2300, the United Nations has forecast.
But the UN Population Division said a small shift in fertility levels could have an enormous impact on population, which is now 6.3 billion.
The nine billion estimate is based on the two-child family, but as little as one-quarter of a child less or one-quarter of a child more per family would result in world populations in 2300 ranging from 2.3 billion to 36.4 billion, it said in a new report called World Population in 2300.
"Clearly, high fertility rates cannot continue indefinitely as they yield extremely large populations in the developing world," said Mr Joseph Chamie, director of the Population Division.
Mr Chamie said the projections are "groundbreaking" because they look so far into the future. Previously, the furthest the United Nation looked forward was 150 years, and instead of making projections by continent, the United Nations did it by country, he said.
Long-range projections are needed by environmental scientists, policy makers, and others who assess the long-term implications of demographic trends, Mr Chamie said. Previous long-range projections put the world population at 10 billion to 12 billion in 2200, he said.
The new projections are lower, largely due to the continuing decline from high fertility rates in the developing world.
The UN report also foresees far more older people. The median age of the world today is 26; by 2300, it will nearly double to 50 years, Mr Chamie said.
PA