At least some of the pregnant women from China caught up in raids this week on so-called maternity hotels, catering to foreign mothers-to-be seeking U.S. citizenship for their babies, are still likely to remain long enough in California to give birth on American soil.

That's because U.S. immigration officials investigating "Chinese birthing houses" for alleged visa fraud, tax evasion and money laundering hope to coax their clients into providing testimony and evidence against operators of those businesses.

Federal officials have declined to say how many pregnant customers they encountered in searches conducted on Tuesday of more than 50 locations suspected of involvement in three large "maternity tourism" networks in Southern California.

No arrests have been made as of Thursday, said Virginia Kice, a spokeswoman for the U.S. Immigration and Customs Enforcement agency, or ICE.

Instead, paying customers of the apartment houses raided in the sweep were taken in for initial interviews by immigration agents, who confiscated their passports and "referred them for more questioning," Kice said.

She declined to say how many if any of the women would end up staying long enough to give birth in the United States now that the schemes were exposed but suggested some were likely to remain in California as the investigation progresses.

"If someone is designated a material witness, their continued presence will be required," she said. "We encountered numerous prospective witnesses at all these locations, not just clients."

She said Mandarin interpreters were assisting in the probe.

The sweep was believed to mark the first such enforcement against an illicit cottage industry that has gained a growing foothold in the United States in recent years while operating largely out of sight of federal authorities.

The U.S. Constitution grants citizenship to any child born on U.S. soil, regardless of parentage, and immigration experts say there is nothing inherently illegal about women from abroad coming to the United States to give birth.

But operators of the maternity hotels are suspected of obtaining non-immigrant U.S. visitor visas under false pretences, as well as failing to report income derived from their services to tax and banking authorities.

Tuesday's sweep targeted operators catering mainly to wealthy Chinese women who paid $10,000 to $80,000 for travel, lodging, medical care and documentation services advertised on the Internet, officials said.

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