(CNN) -- The number of illegal immigrants living in the United States continues to decline from a peak in 2007, according to a report released Wednesday by the Pew Hispanic Center.
The center estimated that in March 2009 -- the most recent figures -- there were 11.1 million unauthorized immigrants in the country, compared to 12 million in March 2007.
"The decrease represents the first significant reversal in the growth of this population over the past two decades," the report states. The report does not discuss what factors could be behind the decrease.
The decrease represents an 8 percent decline from 2007 to 2009 in the number of illegal immigrants currently living in the United States.
According to the report, the most marked decline was in the population of unauthorized immigrants who come from Latin American countries other than Mexico. This group, which includes illegal immigrants from the Caribbean, Central and South America, saw a decrease of 22 percent in the same 2007-2009 period.
In comparison, the Mexican unauthorized population has leveled off at about 7 million since 2007, the report states. Mexicans account for about 60 percent of all illegal immigrants.
The report coincides with other studies showing that while the influx of unauthorized immigrants from Mexico has sharply decreased, unauthorized Mexicans living in the United States are staying put.
According to the report, an average of 150,000 illegal immigrants from Mexico arrived yearly during the March 2007 to March 2009 period, compared to the average of 500,000 that was recorded during the first half of the decade.
For all countries, the average number of new unauthorized immigrants arriving fell from 550,000 a year in 2005 to 300,000 a year in 2009..
The authors of the report point out, however, that despite the decline, the population of unauthorized immigrants was nearly a third larger in 2009 compared to 2000, and it is three times larger than in 1990.
Among the report's other findings was that nearly half of unauthorized immigrants living in the country in 2009 arrived in 2000 or later.
The methodology the authors used took a demographic estimate of the legal foreign-born population in the United States, and subtracted it from the total foreign-born population.
The Pew Hispanic Center is a part of the Pew Research Center in Washington and is an independent research group that does not engage in advocacy or take positions on issues.