The world's population will grow from 7.6 billion this year to 9.8 billion in 2050, with India, Pakistan, and seven other countries accounting for half of that increase, the United Nations has projected.
The countries driving population growth are India, Nigeria, Congo, Pakistan, Ethiopia, Tanzania, the United States, Uganda, and Indonesia, the UN said.
India, which currently has 1.3 billion inhabitants, is expected to replace China, which now has 1.4 billion, as the world's most populous country within seven years.
While fertility levels are declining nearly everywhere, the 47 least-developed countries had high fertility rates averaging 4.3 births per woman between 2010 and 2015, driving their growth.
As a result, 26 countries in Africa are projected to at least double in size by 2050. Nigeria, with the fastest-growing population worldwide, will overtake the United States in size by then, the UN said.
The concentration of population growth in the poorest countries will make it difficult to ensure young people get enough food, health care, education, and other necessities, the UN said.
Meanwhile, populations in Europe, Japan, and other developed countries will continue aging and declining.
Countries where fertility rates are below replacement level include China, the United States, Brazil, Russia, Japan, Vietnam, Iran, Thailand, and Britain.
An influx of refugees and migrants will only partially compensate for dwindling births, the report said.