One in 12 babies born in the U.S. in 2008 were offspring of illegal immigrants, according to a new study, an estimate that could inflame the debate over birthright citizenship.

Undocumented immigrants make up slightly more than 4% of the U.S. adult population. However, their babies represented twice that share, or 8%, of all births on U.S. soil in 2008, according to the nonpartisan Pew Hispanic Center's report.

"Unauthorized...