About 340,000 of the 4.3 million babies born in the United States in 2008  or 8 percent  had at least one parent who was an illegal immigrant, according to a study published Wednesday by the Pew Hispanic Center, a nonpartisan research group in Washington.

Because they were born in this country, the babies of illegal immigrants are United States citizens. In all in 2008, four million children who were American citizens had at least one parent who was in the country illegally, the Pew study found.

Children of illegal immigrants make up 7 percent of all people in the country younger than 18 years old, according to the study, which is based on March 2009 census figures, the most recent data on immigrant families. Nearly four out of five of those children  79 percent  are American citizens because they were born here.

About 85 percent of the parents who are illegal immigrants are Hispanic, the Pew Center said.

The study comes as lawmakers in Washington have been debating whether to consider changing the 14th Amendment, which grants citizenship to anyone born in the United States. The controversy began after Senator Lindsey Graham, Republican of South Carolina, said in July that he might offer an amendment to revoke birthright citizenship for the American-born children of illegal immigrants.