SAN GABRIEL, Calif. – For months, neighbors noticed pregnant Asian women coming and going at all hours at an upscale townhouse development in suburban Los Angeles.

They finally found out that the home was being used as a maternity center for Chinese mothers paying thousands of dollars to give birth in the United States so their children would automatically gain citizenship, city officials said.

The discovery of the center where women stayed before and after delivering their babies at local hospitals was unusual and a possible sign that birthright citizenship is being exploited as a lucrative business, an immigration activist said.

“What this could suggest is … they’re taking it to the next step,” said Mark Krikorian, executive director of the Center for Immigration Studies, which advocates stricter limits on immigration. “Whoever is organizing this type of operation is buying or leasing a home to become a clearing house. That’s a serious problem.”

But it’s not illegal. Women from other countries have long traveled to the United States legally on tourist or student visas and given birth because U.S. law automatically entitles children born on U.S. soil to citizenship.

While some stay under the false assumption that they too can gain citizenship if their child is U.S.-born, many return to their home countries convinced a U.S. birth certificate will afford their child more opportunities in the future.

Often, the women are wealthy and able to pay the steep costs of the trip and medical care.

Krikorian noted that some travel agencies abroad are known to arrange such trips for individuals but not to specialized clinics such as the one in San Gabriel.

Officials in the suburb that’s home to a large Asian population shut down the house for building code violations earlier this month after receiving a complaint about excessive noise, overcrowding and possible building permit violations, said Clayton A. Anderson, the city’s neighborhood improvement services manager.

Inspectors found seven newborns being kept in clear plastic bassinets in a kitchen converted to a nursery.

The three homes, part of a five-unit condo development on a quiet residential street, had adjoining inside walls removed, and rooms were divided so mothers had separate spaces, Anderson said.

Immigration and Customs Enforcement did not plan to investigate because the case did not involve fraudulently obtained visas, agency spokeswoman Virginia Kice said.

Republican lawmakers have moved to limit automatic citizenship for children born in the United States. Earlier this year, they said they hoped to trigger a Supreme Court review of the Constitution’s 14th Amendment or force Congress to take action with legislation they drafted on the issue.

Iowa Republican Rep. Steve King sponsored a bill that would limit automatic citizenship to people with at least one parent who is a citizen, a legal permanent resident or served in the military, but there has been little movement on the legislation since it was introduced.

Last week, Arizona’s state Senate rejected illegal immigration bills that included measures intended to produce a U.S. Supreme Court ruling on who is entitled to U.S. citizenship at birth under the 14th Amendment.

Rep. Judy Chu, a Democrat who represents El Monte in Southern California, said traveling to this country to give birth is not a common practice and defended automatic citizenship for children born in the United States.

“The 14th Amendment is fundamental to the U.S. and too important to change because of the practice of a few,” she said. “It would be a severe disservice to our nation if millions of immigrants are painted with the same brush.”