The Center for Immigration Studies estimates there could be as many as 36,000 births a year to foreign mothers manipulating America’s citizenship laws by visiting the United States just to deliver a child who will then be a U.S. citizen.

A new CIS report released Tuesday looks at the difficulty in collecting data on what’s known as “birth tourism.”

That phenomena made national news in March when federal officials raided a ring of so-called “maternity hotels” in Southern California where pregnant Chinese mothers were allegedly paying birth tourism operations thousands of dollars to have their children in the U.S. where, as citizens, they will be afforded all the rights of an American.

The CIS report, authored by the Center’s research director Steven Camarota, looks at several ways to uncover the annual number of tourism births but notes the problems collecting information.

The Centers for Disease Control, he writes, report that 896,363 women who said they were foreign-born gave birth in 2012.

“If only 2 or 3 percent of these births were to women who are engaging in birth tourism, that would be 18,000 to 27,000 births annually,” he explains in his report. “While this number would be less than 1 percent of the roughly four million annual births in the United States, the aggregate number of birth tourists babies would still be large, especially the cumulative effect over a number of years.”

Another avenue Camarota considers is the number of women who gave an overseas address on their child’s birth certificate. In that case, in 2012 there were 7,955 women in that category. But he notes that there is some evidence that mothers engaged in birth tourism often give the address where they are staying instead of where they live.

“Unfortunately we simply do not know what share of birth tourist mothers provide a U.S. address vs. their overseas address. Further, some share of those providing an overseas address are U.S. citizens returning home to have their child on U.S. soil and are therefore not birth tourists. All of this dramatically reduces the usefulness of the addresses provided with the birth certificate.” he writes.

A final method Camarota considers is looking at Census Bureau’s annual American Community Survey (ACS) which asks women if they had a child in the prior year.

“The survey is designed to reflect the U.S. population as of July 1 of the year the survey was taken, so the survey is recording the number of women living in the country at mid-year who had a child in last half of the prior year and the first half of the year of the survey. In the second half of 2011 and the first half of 2012, the CDC reports 898,975 births to foreign-born mothers,” he writes.

According to Camarota in 2012 the ACS found that there were 863,407 foreign-born women who said they had had a child in the prior 12 months.

“The difference between these two numbers is 35,568 and implies that about 36,000 foreign-born women gave birth in the United States in the 12 months before July 1, 2012, but were no longer in the country,” Camarota explains.

He concluded the report by emphasizing the number as an estimate.