It would be interesting to know what the fertility rate would be without the immigrants, illegal aliens, and their first-generation children.
I did find the following, which is not quite "it" but which is perhaps related:
"Because of the “baby bust” of 1965-1982, there are now fewer non-Hispanic white women in the child-bearing years. White and Asian birthrates are below replacement, black fertility at the 2.1 replacement while Hispanic fertility is 2.99, almost 50% higher than replacement fertility. The large number of Hispanic women in their child-bearing years will increase the number of Hispanic children that are born. "
This is from the Population Resource Center, May 8, 2010.