From Pew Research:
About 275,000 babies were born to unauthorized-immigrant parents in 2014, or about 7% of the 4 million births in the U.S. that year, according to Pew Research Center estimates based on government data. This represented a decline from 330,000 in 2009, at the end of the Great Recession.
Births to unauthorized immigrants accounted for about one-in-three births (32%) to foreign-born mothers in the U.S. in 2014, according to the estimates.
The decrease in births to unauthorized immigrants from 2009 to 2014 contrasts with the trend for the U.S. unauthorized immigrant population overall, which has stabilized. The number of births and the total population both generally rose through the 1990s and 2000s, peaked in 2006 (births) or 2007 (population), and then declined as the recession of 2007-2009 lingered.