The Trump administration has a new target on the immigration front — pregnant women visiting from other countries — with plans as early as this week to roll out a new rule cracking down on "birth tourism," three administration officials told Axios.

Why it matters: Trump has threatened to end birthright citizenship and railed against immigrant "anchor babies." The new rule would be one of the first tangible steps to test how much legal authority the administration has to prevent foreigners from taking advantage of the 14th Amendment's protection of citizenship for anyone born in the U.S.

"This change is intended to address the national security and law enforcement risks associated with birth tourism, including criminal activity associated with the birth tourism industry," a State Department official told Axios.

The regulation is also part of the administration's broader efforts to intensify the vetting process for visas, according to another senior administration official.

The big picture: "Birth tourists" often come to the U.S. from China, Russia and Nigeria, according to the AP.

There's no official count of babies born to foreign visitors in the U.S., while the immigration restrictionist group Center for Immigration Studies — which has close ties to Trump administration immigration officials — puts estimates at around 33,000 every year.

How the new regulation would work: It would alter the requirements for B visas (or visitor visas), giving State Department officials the authority to deny foreigners the short-term business and tourism visas if they believe the process is being used to facilitate automatic citizenship.

It's unclear yet how the rule would be enforced — whether officials would be directed to consider pregnancy or the country of the woman's citizenship in determining whether to grant a visa.

Consular officers who issue passports and visas "are remarkably skilled at sussing out true versus false claims," the senior official said.

"The underlying practical issue is that very few people who give birth in the U.S. got a visa for that specific purpose. Most people already have visas and come in later," according to Jeffrey Gorsky, former chief legal adviser in the State Department visa office.

This is but one step in the administration's plans to make it harder for people from other countries to benefit from birthright citizenship.

"Rome wasn't built in a day," the senior official said. "Just the legal recognition that this is improper and wrong and not allowed is a significant step forward."

The plans to address the use of B visas for birth tourism were included in the latest version of the Unified Agenda of Federal Regulatory and Deregulatory Actions.

Immigration experts expect there to be a similar rule for Customs and Border Protection to go along with the State Department's regulation.

What to watch: Most of Trump's major immigration moves have been met with lawsuits. If the regulation leaves it to officers' discretion to ensure that B visas aren't used for birth tourism, it would be difficult to challenge in court, according to Lynden Melmed, an attorney and former chief counsel at U.S. Citizenship and Immigration Services.

"State Department officials have all the discretion in the world to deny people visas," said Sarah Pierce of the Migration Policy Institute. Foreign nationals who are outside the U.S. and have not yet received visas "don't have a lot of legal standing."

But specific restrictions that could keep out non-birth tourism visitors — such as pregnant women coming to the U.S. for business, etc. — would be legally questionable, according to Melmed and Gorsky.

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