The State Department on Thursday gave visa officers more power to block pregnant women abroad from visiting the United States and directed them to stop “birth tourism” — trips designed to obtain citizenship for their children.

The administration is using the new rule, which takes effect on Friday, to push consular officers abroad to reject women they believe are entering the United States specifically to gain citizenship for their child by giving birth. The visas covered by the new rule are issued to those seeking to visit for pleasure, medical treatment or to see friends and family.

Conservatives have long railed against what they call “anchor babies,” born on American soil and used by their parents to bring in other family members. President Trump has also criticized the constitutional provision that grants citizenship to most babies born on American soil.

It is not clear whether such “birth tourism” is a significant phenomenon or that “anchor babies” do lead to substantial immigration, but many conservatives believe both issues are real and serious. The Trump administration has repeatedly moved to allay conservative immigration concerns, which President Trump has often stoked.

“Birth tourism poses risks to national security,” Carl C. Risch, assistant secretary for consular affairs at the State Department, wrote in the final rule. “The birth tourism industry is also rife with criminal activity, including international criminal schemes.”

Consular officers were already unlikely to grant visa to women who they believed were traveling to the United States solely to give birth. But with the new rule, the White House seems to be signaling to officers abroad that those close to delivering a child would be added to a growing list of immigrants unwelcome in the United States, a list that includes the poor, most refugees and asylum-seeking migrants.

Stephanie Grisham, the White House press secretary, said in a statement that the new rule seeks to stop those who seek “automatic and permanent American citizenship for their children by giving birth on American soil.”

“It will also defend American taxpayers from having their hard-earned dollars siphoned away to finance the direct and downstream costs associated with birth tourism,” Ms. Grisham said. “The integrity of American citizenship must be protected.”

The rule raises the burden of proof for pregnant women applying for tourist visas by outlining in writing that giving birth in the country “is an impermissible basis” for visiting the United States. Even if the women say they are entering the country for medical treatment — a legitimate factor for visa eligibility — an applicant would need to prove that she has enough money to pay for such treatment to the satisfaction of the officer. The woman will also need to prove that the medical care being sought was not available in her home country.

“If an applicant’s responses to this line of questions are not credible, that may give consular officers reason to question whether the applicant qualifies for a visa,” Mr. Risch said in the rule.

The new policy does not change guidance granted to airport officers working for the Department of Homeland Security, meaning visa eligibility changes would occur outside the United States, not at airport immigration counters.

It is also not clear how effective the new rule will be. Some visas allow foreigners to visit the United States multiple times over the course of as many as 10 years, so an applicant could be granted a visa, get pregnant years later and still be permitted to visit the country.

“Unless D.H.S. changes how its officers interpret travel for pleasure to be consistent with the State Department rule, than people will still come to the United States to give birth. This won’t stop that happening,” said Theresa Cardinal Brown, the director of immigration and cross-border policy at the Bipartisan Policy Center.

But those abroad applying for visas close to the delivery date could be denied, she said.