After paying an advance, the customer receives a test kit by an intermediary. Some agents also advise that a nurse be called to take the blood.
The sample should then be sent to Shenzen, a locality on the Hong Kong border in the south-east of the country. There, the smugglers will bring that portion of blood into the former British colony.
None of the intermediaries have revealed how they can get the material into Hong Kong, but everyone assures it will arrive at its destination smoothly.
"They will be transported to the laboratory in a specialized vehicle, and the samples are under proper conditions," said one of the agents, who promised to deliver the test results in 24 hours.
In February, a 12-year-old girl was held at the border carrying 142 jars of blood from pregnant women in her backpack.
"I want a boy"
The test for fetal cells through the analysis of blood from DNA fragments of the fetus represented in 2010 a great scientific advance in allowing to know the sex of the future child from the seventh week, long before the time can be obtained the same information through an ultrasound.
This technique, which also makes it possible to detect the presence of chromosomal anomalies, such as trisomy 21, is less aggressive than amniocentesis, which consists of taking samples of amniotic fluid with a needle.
Instead of sending blood samples, some women prefer the legal option of going directly to the test in Hong Kong. "I have three daughters," a 39-year-old man, who calls himself Wang, told AFP. He accompanies the woman to a test in a laboratory in Kowloon district in the former British colony. "Being sincere, I want a boy," added Wang, who overcame the legal limit on the number of children, as do many wealthy families in the Asian giant.
"The Chinese always want a boy to perpetuate their lineage," he says. "It's archaic, but a lot of people think that way."
If the expectant child is female, the couple will opt for abortion, he says. "My wife is pregnant for less than seven weeks. We can solve this with medicine," he added.
This traffic also questions the practices of Hong Kong's laboratories. By local law, blood tests can be done only with the request of a Hong Kong physician.
While in China it is illegal to export blood samples without special permission in Hong Kong, import is prohibited only in the case of infectious agents.
The Hong Kong Health Department told AFP that since 2016, it has tripled the number of investigations opened by this illegal trade. None of these cases has yet reached the courts.
A laboratory, whose name was cited by one of the intermediaries contacted by AFP, denied that it had already done blood tests on samples received by traffickers and made sure it did not work with intermediaries from mainland China. No Chinese official questioned by AFP responded to the interview requests.