HONG KONG -- By threatening to take away the constitutional guarantee of American citizenship for anyone born in the U.S., President Donald Trump has stirred international unease among couples dreaming of giving birth to children on American soil.

The threat was to revoke the guarantee by executive fiat. But birth tourism enthusiasts remain undaunted. Trump made the statement in the run-up to one of the most heated election campaign seasons in modern U.S. history, one in which his Republican Party used immigration issues to stir up voter anger.

"Perhaps Trump can tighten control over visa issuances," said Echo Wang, a 29-year-old mother in Shenzhen, China. "But I don't think he will be able to change the [constitution]." Wang said she plans to go to the U.S. to give birth to her second child regardless of any executive order Trump might sign.

The congressional midterms were held last week, and the Republicans lost control of the House of Representatives. Some observers say this makes it less likely that the U.S. will end birthright citizenship anytime soon. And viewing the situation from the outside, mothers-to-be remain enamored by the prospect of obtaining U.S. passports for their children.

Wang's daughter was born two years ago -- in Los Angeles. The "birth tour" cost her more than $40,000.

"I want to give my children more options," Wang said. "When my daughter visits Hong Kong, her U.S. passport allows her to enter without a visa. Ironically, we Chinese need a visa [to cross over into] a Chinese territory."

The U.S. is not the only country that offers jus soli, or "right of the soil" citizenship. But the right is rare outside the Americas and rarer still in the developed world. Jus soli means those born in a country are entitled to citizenship regardless of their parents' nationality.

President Donald Trump makes remarks on immigration and border security in the Roosevelt Room at the White House in Washington on Nov. 1. © Reuters

The 14th amendment to the U.S. constitution grants citizenship to "all persons born or naturalized in the United States, and subject to the jurisdiction thereof." Southern states were required to pass the amendment to regain congressional representation after the Civil War. The amendment, adopted in 1868, was meant to guarantee the citizenship of former slaves.

Trump wants to strike down the amendment's Citizenship Clause and said he believes he can do so with an executive order. He revealed his plan on Oct. 30 in an interview with the news website Axios.

The statement was one of many Trump made regarding immigrants ahead of the election and was seen as a tactic to whip up voter anger and drive people to the polls. He has blamed the guarantee for spawning birth tourism and, while campaigning for the presidency, used the pejorative "anchor babies."

"Many come from China," he said. "You'll be surprised -- China now is No. 1."

Ben Cheng Ka-ming, an associate professor a the Hang Seng Management College in Hong Kong, said affluent mainland families are attracted by a number of U.S. benefits, such as educational opportunities. He doubts these families would consider other countries as alternatives.

An attempt to discard birthright citizenship by fiat would also affect overseas graduate students and visiting scholars who chose to have children while studying, Cheng said.

Wealthy families covetous of U.S. benefits for their children have created a lucrative business for agencies who offer one-stop services that include visa applications, hospital reservations and maternity care. Echo Wang arranged for her travel, accommodation, care and hospital needs through one of these agencies.

Dozens of hopeful women in China use a WeChat birth tourism group to trade tips and text about whether Trump will jeopardize their journeys to the U.S. Some advocate lying about the purpose of their visit; others plan to enter the country through tourist-friendly Las Vegas or Hawaii, which are viewed as having laxer controls.

Cici Liu, a Chinese partner of the Yueyue Center, a Houston-based maternity care center, said she has received more queries from mainland families since Trump said he could do away with birthright citizenship. The threat, however, has had no effect on business.

"Families from the Chinese side are more concerned, but our American partners said there shouldn't be any problem," Liu said in Beijing, where she operates. "They say it will take at least a few years for Trump to change the constitution."

Liu expects some prospective mothers to move up their schedules to beat any possible deadline.

If Trump is using the issue to pick a fight, it will take years to settle. "You cannot end birthright citizenship with an executive order," Paul Ryan, the outgoing speaker of the House of Representatives, said in a radio interview.

In a country of laws, striking down the Citizenship Clause requires another constitutional amendment, but there has not been one since 1992.

Furthermore, presidents are not involved in the amendment process. The Senate and House of Representatives must begin the process by passing a joint resolution by a two-thirds majority vote. The resolution then must be ratified by three-fourths of state legislatures, or 38 of 50.

The process can be contentious (Remember that the Citizenship Clause came about due to the Civil War.) and take years. Since Democrats now control the House, the only hope Republicans have of starting the ratification process is by making a wedge issue out immigration in general and jus soli specifically -- and by using it to cudgel Democrats.

That is not out of the question, considering how the Republicans used the immigration debate to rile up their voters and get them to the polls this week.

According to World Atlas, 30 countries, mostly in the Americas, allow right-of-the-soil citizenship. Canada is among them, but a conservative opposition party is eager to end the practice. This seems to be a trend. Since 2000, the guarantee has been abandoned by a number of countries, including New Zealand, Ireland, the Dominican Republic, India and Australia.

It is difficult to grasp the exact number of birth tourists. In 2016, the Pew Research Center issued a report on the number of unauthorized immigrants giving birth in the U.S. "About 275,000 babies were born to unauthorized-immigrant parents in 2014, or about 7% of the 4 million births in the U.S. that year," the article says.

In 2015, the Center for Immigration Studies, which calls itself an independent, nonpartisan, nonprofit research organization, estimated that the U.S. receives 35,000 birth tourists a year. And last week, the South China Morning Post received an estimate from a birth tourism agency that 80,000 Chinese mothers had American babies in 2016.

Trump's threat came as music to the ears of birth tourism agencies in Canada.

"We have received a lot of inquiries from Chinese would-be mothers who intended to give birth in the U.S. but are now considering Canada instead," said the Global Baby-Mom and Baby Care Centre in a post on Sina Weibo, a Twitter-style social network. "It is absolutely possible to give birth in Canada and earn Canadian citizenship for your baby."

The post appeared on Oct. 31, the day after Trump made the threat, and has received more than 15,400 likes.