The U.N. estimates that the world's population will pass the 7 billion mark on Monday.

Much of that growth has happened in Asia — in India and China. Those two countries have been among the world's most populous for centuries. But a demographic shift is taking place as the countries have modernized and lowered their fertility rates. Now, the biggest growth is taking place in sub-Saharan Africa.

Due in part to that region's extreme poverty, infant mortality rates are high and access to family planning is low. The result is high birth rates and a booming population of 900 million — a number that could triple by the end of the century. Population expert Joel Cohen points out that, in 1950, there were nearly three times as many Europeans as sub-Saharan Africans. If U.N. estimates are correct, there will be nearly five sub-Saharan Africans for every European by 2100.

As NPR's Adam Cole reports, it was just over two centuries ago that the global population was 1 billion — in 1804. But better medicine and improved agriculture resulted in higher life expectancy for children, dramatically increasing the world population.

As higher standards of living and better health care are reaching more parts of the world, the rates of fertility — and population growth — have started to slow down, though the population will continue to grow for the foreseeable future.

U.N. forecasts suggest the world population could hit a peak of 10.1 billion by 2100 before beginning to decline. But exact numbers are hard to come by — just small variations in fertility rates could mean a population of 15 billion by the end of the century. -NPR