Sheng Hailin, 60, gave birth to twins through in vitro fertilization. GUO CHEN/XINHUA

If the parents supply their own sperm and ova, the center charges $110,000 to $160,000, which includes the payments to the surrogate mother, the agency's fee, insurance and legal bills.

Generally speaking, overseas parents are required to travel to the US three times. First, they will visit the center for a one-on-one consultation with lawyers and surrogate mothers. Contracts are signed after a decision is made.

"We have a big pool of surrogate mothers of different races, including Asian," Liu said. "But usually, I recommend clients choose a white or black surrogate mother because they are stronger and have thicker uterine walls."

The second visit is mainly for the in vitro fertilization process. During the pregnancy, the center will check the physical and mental condition of the surrogate mother every week and update the clients on her condition.

"If required, we can also arrange video calls between surrogate mother and intended parents," she said.

"Ten months after that, the clients fly to the US for the third time and take their lovely baby back. However, travel and accommodation costs are not included in our package.

"About 80 percent of our clients turned to surrogacy in the US not only with the consideration of being protected by the law but also because the child will be recognized legally as a US citizen. Moreover, the much higher success rate of IVF in the US is also crucial to them since many Chinese female clients are not at their best age for childbearing," Liu said.

Compared with the success rate of IVF in China's top fertility hospital, at 30 to 50 percent, the rate in the US is as high as 83 percent, Liu said.

The Tangs had left the office in Beijing after about an hour, but were still concerned about the center's reliability.

"We should pay a visit to California, at least to make sure the center is there and is legally protected," Tang said.

Infertility issues

An increasing number of couples in China have been facing infertility in recent years. A report from the China Population Association showed that the infertility rate among people of childbearing age rose from 3 percent in 1992 to 12.5 percent in 2012, equivalent to 40 million people.