The United Nations Population Fund projected in its most recent report that world population would reach 9.6 billion in 2050. The U.N. also predicts that 43 countries will experience population decline through 2050. It projects that India, China, Nigeria and the United States will have the highest populations in the world in 2050, each having at least 400 million people.

Growth in Developing Countries

The U.N. predicts that most of the world’s population growth will take place in high-fertility countries and places that already have large populations. In its “World Population Prospects” report for 2012, the U.N. noted that “almost all” of the additional 3.7 billion people added to the world by 2050 will be in developing countries. The total population of developing countries will rise to 8.2 billion in 2050. The 49 least developed countries are expected to double their populations by 2050, from 900 million to 1.8 billion.

India & China

The U.N. report “World Population Prospects: the 2012 Revision” projects that India will surpass China as the world’s most populous country just after 2028, when both countries hit 1.45 billion people. India will take over China because its population will continue to grow while China’s population contracts. India will have a total of 1.652 billion people by 2050, according to the Population Reference Bureau’s “2013 World Population Data Sheet.” China will be the second largest country in 2050 with 1.314 billion people.

African Countries

Wendy Baldwin, president and CEO of the Population Reference Bureau (PRB), said that “nearly all” of the world’s population growth will be in African countries between 2013 and 2050. The PRB estimates that Africa’s population will more than double from 1.1 billion to 2.4 billion. The birth rate in Sub-Saharan Africa is 5.2 children per woman and 43 percent of the region’s population is under 15 years old. Nigeria’s population will be the biggest in Africa in 2050, when it overtakes the United States as third most populous country in the world with 440 million people.

United States

The United States will increase its population from to 400 million by 2050, yet, due to extraordinary growth in Nigeria, the U.S. will move from the third largest population in the world to the fourth largest. The PRB researchers said that rural-urban immigration is a large part of the demographic story over the past several decades and into the future. The United States has three cities that made their list of the 30 largest metropolitan areas in the world, including New York, Los Angeles and Chicago.