The Census Bureau said the world's population will reach 7 billion in 2012.

According to projections released Thursday, there are about 6.7 billion people in the world today. China and India have the largest populations, followed by the U.S. with 304 million.

The world's population hit 6 billion in 1999, meaning it will take only 13 years to add a billion people.

By comparison, a demographer at the Population Reference Bureau said the number of people didn't reach one billion until 1800. Carl Haub said the quick climb comes from rapid population growth in developing countries, where medical and nutritional advance led to a population explosion after World War II.

But with more women in those countries going to school and joining the work force, the growth rate is starting to slow worldwide.