Wang Mei stood in front of an immigration officer at Los Angeles International Airport, trembling inside, as she described it later. She was wearing a long black dress with a coat over it, hoping that this would conceal from the officer the seven-month old baby growing inside her. If he spotted the little one, would he send her back to China?

Madame Wang is one of the hundreds of mainland mothers who have gone to the United States to have their babies, enabling the infant to become a US citizen. Under a law passed in 1868, any child born on American soil is a US citizen, regardless of who his or her parents are.

This has spawned a flourishing industry at home and in major cities like New York, Los Angeles and San Francisco, providing a comprehensive service to these mothers, from arranging visas and plane tickets to high-class accommodation before and after the delivery and medical care. One popular translation of San Francisco is 'Gold Mountain', a reference to the gold rush of the 1850s, when thousands of Chinese emigrated to California.

Going to the US to have a baby is a more expensive and ambitious alternative than coming to Hong Kong. Of the 60,000 births in the SAR last year, at least one-third were to mainland mothers. The cost is about HK$100,000. Many are from the same social class who want a delivery in the US – rich, well-educated and upwardly mobile – and for the same reasons: they want high-quality care, their child to have the right of residence, free education and social benefits offered by Hong Kong and the US and the option of more than one child.

In Hong Kong, the births are a controversial issue if they are in public hospitals. Last October, the Hospital Authority suspended bookings by mainland mothers for three months because a flood of bookings had filled many of the public hospitals; in the first eight months of 2009, non-local women accounted for about 25 per cent of deliveries in public hospitals.

The authority was responding to fears among local women that beds would not be available when their pregnancies came to term. In 2007, the government implemented a pricing scheme that charges mainland women far more than local who give birth in public hospitals: to book a bed in a public maternity ward costs HK$39,000.

This increase in price is one factor pushing mainland mothers to go to the US and why the number going there is increasing.

"The average cost, including a three-month stay, all medical fees and air tickets, is about 120,000 yuan," said Andy Wei, who describes himself as 'host' of USNewBorn, which runs two centers in the US "Currently, the number of Taiwan mothers is more than that of mainland mothers, due to visa problems and access to information. By 2011 or 2012, the number of mainland mothers will overtake that of Taiwan mothers. The mainland mothers need more English support and do more shopping."

For Madame Wang, the story had a happy ending. She made it through the immigration and her son was born in a birth centre in Los Angeles in early 2008. He is now two and half years old and living happily in the family home in Jinan, Shandong province.

"As an American, my son will have visa-free access to 180 countries, enjoy 13 years of free education and we will save one million yuan on his university fees there," she said. "He will enjoy social welfare benefits, including a high-quality old people's home and, if he is in another country hit by turmoil, will receive preferential treatment in boarding the plane at an airport to leave. The investment return is much higher than putting the money in a bank."

Wang works in a trading company in Jinan. She is among a small elite, in terms of money, attitude and organization, who take the significant step of flying 15 hours across the Pacific Ocean, far from home and family, to have their child in a foreign land. Those in the business say about 500 mainlanders go each year, entering on tourist or business visas. They are business executives, doctors, lawyers, accountants and other professionals, of whom a third have lived or studied abroad and a proportion speak English.

Another mother was Liu Qing from Shanghai. Originally, she had no idea of having a second child or going to the US. But when she found she was pregnant a second time, she was in a dilemma because of China's strict one-child policy. She could hide at home or go and live with relatives in the countryside until the delivery. But if the local family planning bureau found, she and her husband ran a risk of a fine up to three years of their income and other possible sanctions.

"We might as well as pay this money and have the baby abroad. My husband had studied abroad and decided quickly that, if we could not have the child at home, we would have it in the US."

There are other motives – dislike of the mainland's education system and a future exit strategy in case things go wrong at home, either for the parents or the government, and future advantages in employment as well as education. After the child reaches the age of 21, the parents can apply to emigrate to the US as the parents of an American citizen.

Jin Zhong, editor of Kaifang (Open) magazine, said that, for some families, the birth of a child in the US is part of a contingency plan in case of trouble at home. "The rich buy homes abroad, put money there and obtain permanent residence there, so that they can leave if they need to at short notice."

The most popular choice is Los Angeles, which has 30 birth centers, which are like five-star hotels, offering guests a private room with bathroom, food, transport, medical examinations, language lessons and shopping expeditions.

Most of the doctors are Chinese-Americans who deliver the baby at the centers. Only if there are complications do they take the mother to a hospital. The centers were first established to cater to Taiwanese mothers, many of whom gave birth in the US so that, as foreigners, their sons would not have to do the military service compulsory in the island.

Madame Wang's anxieties at Los Angeles were unfounded, said Andy Wei. "Current immigration law does not forbid pregnant women to enter the US. It would be discrimination. On the other hand, having the baby in the US does not bring loss but profit to the U.S. – the rich couple will pay all the bills and their child will pay American tax all his life."

Another downside is that, since in China the child will be a foreigner, the parents will have to pay higher school fees than for a local student.

One of Wei's competitors is Chinese Baby Centre, with offices in Beijing and Shanghai and a 'five-star birth centre' in Los Angeles.

"We are mainlanders who have grown up in China and understand the national character and policies of the mainland better than Taiwan and Hong Kong people," said its website. "We provide services which Taiwan and Hong Kong agencies cannot provide. We treat every customer as a friend and help to provide a common path for them among the Chinese community of Los Angeles.

"Our Chinese founder, Madame Hao, was among the first mainland mothers to have her baby in the US and found most of the birth centers in the Los Angeles region were operated by Hong Kong and Taiwan individuals but that there were no companies involved. So she decided to set up a company and, in recent years, more than 100 families have followed her example," it said.

The website promises the highest level of service from its staff, so that each customer "could deeply understand the US and enjoy the real life of white Americans." Vicky Wong, its Shanghai representative, declined to answer questions on the telephone.

In China, more than 100 agents compete to provide mothers to the birth centers, with commissions ranging from US$500 to US$1,000. Some are individual operators, others work directly for the centers or are middlemen.

Many Americans object to this policy of giving nationality to children, neither of whose parents are citizens. They want the government to follow the example of New Zealand, which passed a law in January 1, 2006, limiting citizenship to babies of whom at least one parent is a New Zealand citizen or lawful permanent resident. Since the late 1990s, Republicans have introduced similar legislation in Congress; but it has not been passed and seems unlikely to be.

According to the Pew Hispanic Centre, a research body, of the 4.3 million babies born in the US in 2008, 340,000 were children of illegal immigrants. Between 2000 and 2006, the number of non-Americans born in the US increased by 53 per cent, compared to an increase of five percent in the number born to Americans.

Mothers come from countries such as Turkey, South Korean, Mexico and other Latin American countries, as well as Taiwan and China.