(CNN) -- A two-year decline in the unauthorized immigrant population in the United States has leveled off, at least for now, a Pew Hispanic Center report released Tuesday found.
The leveling off of what had been a statistically significant decline from a peak in 2007 coincides with a slight improvement in the U.S. economy, one of the study's authors, Jeffrey S. Passel, told CNN.
In March 2010, there were 11.2 million unauthorized immigrants in the United States, compared with 11.1 million in 2009, the study estimates. Statistically, the change was insignificant, as the numbers are virtually the same. The peak year for unauthorized immigrants in the country was 2007, with an estimated population of 12 million.
"We saw a large drop (from 2007), but it doesn't look like that drop is continuing," Passel said.
The number of unauthorized immigrants in the work force in 2010 was estimated at 8 million, a slight increase from the year before, but again, not a statistically significant shift, according to the report. That number is equivalent to about 5.2% of the work force, the study says.
While the state of the economy is known to have an effect on the numbers of people entering and leaving the country illegally, it is difficult to tell from the data -- based on Census Bureau figures -- what the reasons are behind the leveling off.
"We're kind of struggling on how to answer those questions ourselves," Passel said.
From the census data, researchers can estimate how many people are living illegally in the United States, but no one is asked the reasons why they came or why they are returning to their homelands.
However, researchers compare the changes in the unauthorized immigrant population with other factors believed to be related.
For instance, as the number of unauthorized immigrants shrank from 2007, so did the number of unauthorized immigrants from Mexico. Unauthorized Mexicans, who make up the largest segment of illegal immigrants, fell from 7 million in 2007 to 6.5 million in 2010, the study states.
"The decline in the size of the unauthorized immigrant population from its peak in 2007 appears to be driven mainly by a decrease in the number of such immigrants from Mexico," the study states.
The rebounding economy may be a counteracting force that kept the 2010 figures flat, Passel said.
"What we've seen historically is that when employment opportunities have increased, we've seen increased flows" to the United States, he said.
The economy in 2009 was better than in 2008, and indications are that it will be even better in 2010, hinting that illegal immigrations numbers could potentially creep back up, he said.
The study also estimates that in 2010, there were 350,000 children born in the United States to at least one parent who was in the country illegally. The figure was essentially unchanged from 2009.
Some states saw statistically significant shifts in their unauthorized immigrant population. Florida, New York, Colorado and Virginia, which in recent years had drawn many illegal workers, saw decreases in that segment, the study found. Arizona, Utah and Nevada combined also saw a decrease.
In contrast, Louisiana, Texas and Oklahoma combined saw a significant increase, the study says.
The Pew Hispanic Center is a nonpartisan research organization that does not take positions on policy issues.