A Chinese-American lawyer has been sentenced to 21 months in federal prison for obstructing justice related to a scheme in which he agreed to help a witness in the "birthing centers" investigation flee the United States.

Ken Zhiyi Liang, 39, an Irvine, California-based immigration attorney, was sentenced on Monday afternoon by US District Court Judge Andrew J. Guildford. He was convicted in September of conspiring to obstruct justice, obstructing justice, and tampering with a witness.

Liang was arrested on May 15 last year after accepting $6,000 from a Chinese woman who had been designated a material witness in a criminal investigation into "maternity hotels" operating in Southern California, in exchange for helping her abscond to China.

The hotels are businesses that charge a fee for Chinese women to travel to the US to give birth so that their children become American citizens. In March, the federal agents raided more than 30 such centers to collect evidence of alleged visa and tax fraud.

Twenty-nine Chinese persons were designated as material witnesses to testify against the business operators. Federal arrest warrants were issued for 10 of them in May after they had jumped bail and returned to China in April.

As a material witness, the woman implicated in Liang's case was subject to a court order preventing her from leaving the US. Liang had represented the witness until the court removed him as attorney of record on April 17, 2015.

Unbeknownst to Liang at the time, the woman was cooperating with federal agents with from US Immigration and Customs Enforcement's Homeland Security Investigations (HSI), who were monitoring conversations between Liang and the witness.

During the conversations, Liang outlined a plan in which he would assist the witness by having her board a commercial airliner in the United States without travel documentation, so she could travel to China undetected by federal authorities.

Liang was caught on hours of video and audio recordings selling and marketing his abilities to the woman to help smuggle her out of the US, in violation of court orders, in exchange for a $6,000 fee for himself and a $1,500 to $3,000 fee for three co-conspirators who would assist him, prosecutors wrote in a sentencing brief filed with the court.

He returned the $6,000 he had accepted after he was arrested while walking with the witness to meet the three co-conspirators in a coffee shop in Corona, California.

"As an attorney and officer of the court, Mr Liang owed a heightened duty to respect and follow court orders," said US Attorney Eileen M. Decker. "Instead Mr Liang chose to violate those court orders, and induced others to violate court orders, for his own personal profit. Today's sentence is a reminder of the importance of court orders and protecting the integrity of federal investigations."

According to court documents, Liang provided assistance to two other material witnesses who fled to China and to another material witness who was intercepted at Los Angeles International Airport on April 15.

The investigation into the birthing centers is being conducted by HSI and IRS – Criminal Investigation.