The number of illegal immigrants in the country is stabilizing at just over 11 million, as a troubled U.S. economy doesn't hold the same promise of jobs that it did in more abundant times, according to a new study.

As of March, 11.2 million unauthorized immigrants were living in the U.S., which was virtually unchanged from a year earlier, according to new estimates from the Pew Hispanic Center. This stability in 2010 follows a two-year decline from the peak of 12 million in 2007 to 11.1 million in 2009 that was the first significant reversal in a two-decade pattern of growth. Unauthorized immigrants were 3.7 percent of the nation's population in 2010.

The number of unauthorized immigrants in the nation's workforce, 8 million in March 2010, also did not differ from the Pew Hispanic Center estimate for 2009. As with the population total, the number of unauthorized immigrants in the labor force had decreased in 2009 from its peak of 8.4 million in 2007. They made up 5.2 percent of the labor force in 2010.

The number of children born to at least one unauthorized-immigrant parent in 2009 was 350,000 and they made up 8 percent of all U.S. births, essentially the same as a year earlier. An analysis of the year of entry of unauthorized immigrants who became parents in 2009 indicates that 61 percent arrived in the U.S. before 2004, 30 percent arrived from 2004 to 2007, and 9 percent arrived from 2008 to 2010.

Other key points from the new report include:

The decline in the population of unauthorized immigrants from its peak in 2007 appears due mainly to a decrease in the number from Mexico, which went down to 6.5 million in 2010 from 7 million in 2007. Mexicans remain the largest group of unauthorized immigrants, accounting for 58 percent of the total.

• The number of unauthorized immigrants decreased from 2007 to 2010 in Colorado, Florida, New York and Virginia. The combined population in three contiguous Mountain West states-Arizona, Nevada and Utah-also declined.

• In contrast to the national trend, the combined unauthorized immigrant population in three contiguous West South Central states-Louisiana, Oklahoma and Texas-grew from 2007 to 2010.

• Although the number of unauthorized immigrants in the U.S. is below 2007 levels, it has tripled since 1990, when it was 3.5 million and grown by a third since 2000, when it was 8.4 million.

The deep recession that began in the U.S. economy officially ended in 2009, but recovery has been slow to take hold and unemployment remains high. Immigration flows have tended to decrease in previous periods of economic distress.