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The number even exceeded the figure for 1910, the peak year for the last great wave of immigrants, said the report from the private Center for Immigration Studies, which favours restrictions on immigration.
In 2002, 23 percent of all births were to immigrant mothers -- both legal and illegal -- compared to 15 percent in 1990 and six percent in 1970. Ten percent of births were to illegal immigrants. Steven Camarota, the center's director of research who compiled the report, said the evident surge in immigration was bound to have an enormous impact.
"Research shows that one of the biggest challenges immigrant-receiving countries face is the assimilation of the children of immigrants," said Camarota.
"With immigrants accounting for such a large, and growing, share of births, America is headed into uncharted territory. We simply don't know how these children will assimilate, but it is clear that the stakes for America are enormous," he said.
The report found that more than half of the births to immigrant mothers in 2002 were to Hispanic immigrants. No single cultural or linguistic group has ever accounted for such a large share.
The report's data was compiled from birth records collected by The National Center for Health Statistics. While not contradicting the figures on immigrant births, pro-immigrant groups took exception to Camarota's conclusions, particularly his warning about non-assimilation.
"Sociologists call the US the language graveyard, because people do come here speaking their native tongue and then the kids quickly learn English," said Angela Kelley, deputy director of the National immigration Forum.
Kelley said the one area of shared concern was over the level of illegal immigration.
"That's the debate to be had -- how do we manage immigration intelligently, not whether people are having babies who aren't going to learn English," she said.