"The Center for Immigration Studies has an interest and a history of basing these estimates on assumptions which suit their interests," she said. "There is a high likelihood that there is some inflation of numbers here."

Jeffrey Passel, a demographer with the Pew Hispanic Center, a non-partisan research group, said the calculations are similar to previous studies by other organizations, but the percentage of births to illegal immigrant mothers seemed "a bit high."

Passel also said the number of children of immigrants has been increasing slowly for the past 10 to 15 years in large part because immigrants tend to be of child-bearing age.

The Center for Immigration Studies' statistics are based on birth certificates from all 50 states and the District of Columbia. The National Center for Health Statistics, which compiles the data, reports that more than 99 percent of births in the United States are registered, the study said.

The calculation for illegal immigrant births was made by using a formula that took into account the total foreign-born population, the fertility rates of female immigrants and estimates of illegal immigrants in the United States.