WASHINGTON — Nearly 15 percent of all births in Utah are to immigrant mothers, a rate that has increased dramatically in the past 35 years, according to a new study.

Of those births, an estimated 47 percent are to mothers who are illegal immigrants, according to the study released this week by the Center for Immigration Studies, a Washington think tank that advocates stronger immigration controls.

The study's result: 7 percent of Utah's births in 2002 were to illegal immigrant mothers.

"The numbers are really, really striking," said Steve Camarota, director of research at the Center for Immigration Studies and author of the study.

Hispanic and immigrant advocate groups decried the study as a partisan attack on immigrants and a veiled criticism of the 14th Amendment to the Constitution, which guarantees birthright citizenship.

The study found that, in Utah, births to immigrant mothers spiked from 3.7 percent in 1970 to 14.7 percent in 2002.

Nationwide, nearly a quarter of all births are to immigrant mothers — a record higher than during the peak of the previous great immigration wave in 1910, the study said.

It concludes that nearly 10 percent of children born in the United States are born to illegal immigrant mothers.

In 2002, it found that 23 percent of all American births were to immigrant mothers, both legal and illegal. Of those children, 45 percent were born to Mexican mothers — the largest share of births to one immigrant country of origin in history. In 1910, 22 percent of U.S. children were born to immigrant mothers.

"We are heading, if you will, into uncharted territory," Camarota said. "In the past, immigration was significantly reduced when it reached a similar level, but that's not happening today."

Camarota that the number of births to illegal immigrant mothers makes enforcement of immigration laws difficult.

Since those children are automatically U.S. citizens, they can stay permanently and could be a factor in keeping their parents from deportation. The study concludes that the problem of illegal immigration will only get worse if left unchecked.

Cecilia Munoz, vice president for policy at the National Council of La Raza, a Hispanic civil rights group, said the study "is being presented in the most inflammatory possible way by an organization which is anti-immigrant, so it's important to read it with that understanding."

Munoz also questioned its calculation of illegal immigrant mothers, a population that is, by definition, difficult to count.

"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."