Federal and state governments do not specifically track the annual number of U.S. births by illegal immigrants. But according to the conservative Center for Immigration Studies, one out of every 10 births in the U.S. is to an illegal immigrant.
The number of U.S. births by non-resident mothers -- women who are in the U.S. temporarily -- has also been on the rise: up 53 percent between 2000 and 2006, according to the National Center for Health Statistics. Total births rose 5 percent in the same period.
It's unclear how many of the 7,600 births to non-resident mothers in 2006, the most recent data, were by women who traveled to the U.S. with the sole purpose of obtaining citizenship for their child. The count includes international travelers passing through and foreign students studying at U.S. universities.
The State Department and Department of Homeland Security have no specific regulations banning pregnant foreigners from entering the United States. But officials say they can and do turn away pregnant women with obvious designs on coming to the United States to take advantage of free medical care.
"When determining if an individual will be allowed to enter the U.S., Customs and Border Protection officers take into consideration the date the child is due for delivery and the length of time the individual intends to stay in the U.S.," a Department of Homeland Security spokesman said.
The 14th Amendment to the Constitution, which was enacted after the Civil War to grant citizenship to descendants of slaves, reads, "All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the state wherein they reside."
"I find it so interesting that strict constitutionalists, whether it's Kyl, McCain, Brewer, the Tea Party, find that the Constitution can be malleable if it fits their ads," charged Democratic Rep. Raul Grijalva of Arizona. "You're either for the Constitution that you swore to uphold or you`re not. It's politics. It's useful, destructive politics that they're using. And they have to continue to use it until the midterms."
"I don't think the founders understood when they did the 14th amendment that they would create a circumstance where people could fly into America, all over the world and have a child and that child would have dual citizenship, fly back to their home countries," said Republican Sen. Jeff Sessions. "It has been clearly abused."
The Supreme Court has only addressed the issue once, clarifying in 1898 that citizenship does apply to U.S.-born children of legal immigrants who have yet to become citizens.
The United States is one of the few remaining countries to grant citizenship to all children born on its soil. The United Kingdom, Ireland, India and Australia, among others, have since revised their birthright laws, no longer allowing every child born on their soil to get citizenship.
ABC News' Avery Miller contributed to this report.