President Donald Trump’s plan to end birthright citizenship through executive order—if it holds up—could save taxpayers billions and significantly reduce future illegal immigration, according to estimates from the government and an immigration-focused think tank.

An 1898 Supreme Court ruling helped shape the modern interpretation that the 14th Amendment means any child born on American soil is a citizen. However, many legal scholars dispute the interpretation.

The dispute comes down to whether illegal immigrants fall under those who are “subject to the jurisdiction of the United States.”

Birthright citizenship and an executive order to address it raise a host of legal, policy, and constitutional questions that are debatable.

Here are the facts to understand the size and scope of the issue.

1. Totals and Projections

About 4.5 million U.S. citizens have “at least one inadmissible or deportable parent” in the country, according to a 2017 report by the Congressional Budget Office.

Once the citizen children of these illegal immigrants become adults, they are allowed to sponsor their parents for legal status, as well as family members living abroad to come into the United States on a visa.

These children with parents who are illegal immigrants are eligible for all federal benefit programs, but “their parents’ immigration status can affect the likelihood that their parents would choose to enroll them in those programs,” according to the CBO.

The CBO estimates that 60,000 more citizen children will be born to illegal immigrant parents over the next decade.

The population of children born in the U.S. to illegal immigrant parents grew from 2.7 million in 2003 to 4.5 million in 2010, Mark Krikorian, executive director of the Center for Immigration Studies, a pro-enforcement think tank, told a House Judiciary subcommittee in 2015.

In 2014, 1 in 5 births in the U.S. were to an immigrant mother, legal or illegal, according to the Center for Immigration Studies. Of those, 297,000 births were to illegal immigrant mothers.

CIS, which issued a study on immigrant births earlier this month, used data from the Census Bureau’s American Community Survey collected between 2012 and 2016.

“The 297,000 births per year to illegal immigrants is larger than the total number of births in any state other than California and Texas,” the CIS study says. “It is also larger than the total number of births in 16 states plus the District of Columbia, combined.”

A 2010 Pew Research Center study, relying on 2008 Census Bureau data, determined that one out of every 12 newborns in the United States is born to an illegal immigrant.

2. Areas Most Affected

The bulk of illegal immigrant births occur in eight states, according to the CIS study, which covered 2014.

The largest numbers, not surprisingly, were in the three most populous states that are also border states. California had an estimated 65,000 births to illegal immigrant mothers, followed by Texas with about 51,000, and Florida with about 16,000.

Next come Illinois with an estimated 14,000 births to illegal immigrant mothers, Georgia with 13,000, New York with 12,000, and New Jersey and North Carolina with 11,000 each.

The 28,000 births to illegal immigrant mothers in the Los Angeles metropolitan area alone is larger than the number of births to illegal immigrant mothers in 14 states and Washington, D.C., according to the study.

Nevada had the highest share of births to illegal immigrant mothers of any state, with about 1 in 6, the study says. That compares to 1 in 7 births in the larger California and Texas.

As for metropolitan areas, the estimated 28,000 births to illegal immigrants in greater Los Angeles is larger than the total number of such births in 14 states and Washington, D.C.

More than 1 in 7 births to illegal immigrants occur in the greater metropolitan areas for Los Angeles, Las Vegas, San Jose, Dallas, and Houston.

The study adds: “Typically between two-thirds and three-fourths of these births are likely paid for by taxpayers.”

3. Birth Tourism

Birth tourism is a separate issue. It often involves expecting mothers who legally enter the United States so their children will be born as U.S. citizens.

About 36,000 “birth tourist” mothers come to the United States each year to have such children, according to another report by the Center for Immigration Studies in 2015.

The Los Angeles Times reported the same year that the practice of birth tourism “has become particularly popular in recent years with the newly wealthy Chinese middle class.”

The practice also reportedly has become increasingly common in Turkey and Nigeria.

Federal authorities have cracked down on birth-tourism rings, in which some mothers paid $80,000 to birth their children in the United States.

4. What Other Countries Are Doing

Birthright citizenship is rare among other countries, but the United States is not unique, as Trump stated in an interview with Axios.

“We’re the only country in the world where a person comes in and has a baby, and the baby is essentially a citizen of the United States,” Trump told Axios.

Only two advanced countries, as classified by the International Monetary Fund, have birthright citizenship. Canada is the other.

In all, more than 30 of the world’s 194 countries, almost all of them in the Western Hemisphere, have some form of birthright citizenship.

No European country provides automatic citizenship to children of illegal immigrants. Great Britain did away with the practice in 1983, and Ireland did so in 2004.

Australia ended the practice in 1986, India in 1987,and New Zealand in 2006. The Dominican Republic is among the most recent countries to end the policy, doing so in 2010.