A U.N. report says the world's population will reach the 7 billion mark Monday.

The report looks at the 7 billion milestone as a sign of people living longer and improved birth survival rates. Experts say the U.N. report displays the disparities between countries.

One of the study's authors, Richard Kollodge, says many low-income countries of sub-Saharan Africa and some in Asia have seen a growth in population that is outpacing economic growth rates.

Asia's population of 4.2 billion residents ranks as the world's most populous region.

Africa's growth rate is expected to triple to 3.6 billion people by the start of the next century.

Fertility rates in many European nations and Japan have dropped below the replacement rate, causing concern that shortages of labor and productivity could threaten the quality of life.