UNITED NATIONS (AP) – The world’s population will hit 7 billion early in 2012 and top 9 billion in 2050, with the vast majority of the increase coming in the developing countries of Asia and Africa, according to a U.N. estimate released Wednesday.

Hania Zlotnik, director of the U.N. Population Division, said that “there have been no big changes” from the previous estimate in 2006.

“We are still projecting that by 2050 the population of the world will be around 9.1 billion,” she said at a news conference. “The projections are based on the assumption that fertility that is now around 2.56 children per woman is going to decline to about 2.02 children per woman in the world.”

Zlotnik said if fertility remained about where it is now, then world population would reach 10.5 billion by 2050. If fertility fell even more than expected, to about 1.5, then the population would only increase to 8 billion by mid-century, she said.

Population growth will remain concentrated in the most populous countries through 2050. Nine nations are expected to account for half the projected increase: India, Pakistan, Nigeria, Ethiopia, the U.S., Congo, Tanzania, China and Bangladesh, the report said.

In sharp contrast, the populations of 45 countries or regions are expected to decline at least 10 percent over the same period, including Japan, Italy and many other countries that were once part of the Soviet Union, the U.N. said.

According to the study, the largest number of migrants will head to the United States – an estimated 1.1 million every year between 2010 and 2050.

The immigrants and the U.S. birth rate will help boost the U.S. population from an estimated 314.7 million in mid-2009 to 403.9 million in 2050, according to Gerhard Heilig, chief of the U.N.’s Population Estimates and Projections Section.