Major General Ho Sy Tien, director of the Criminal Police Department with Vietnam's Ministry of Public Security, said they believed the girl was most likely abducted and sold across the border sometime between 2010 and 2011.
She would have been around six or seven years old at the time.
"So far there have been no results. We have yet to find her family, though initial findings of the case have been published in the media, so anyone who may know her can inform the police," he said.
Distressing photographs released last Friday show the little girl's huge baby bump and stretch marks.
She was taken to hospital with the 35-year-old man and a woman who claimed to be the girl's mother-in-law.
They said the girl was three months pregnant and that she needed a routine test to confirm that her foetus was healthy.
Medics immediately called the police when the two adults tried to pass off the girl as a 20-year-old woman.
She couldn't speak Mandarin, meaning doctors were unable to question her about her age.
A staff member said: "She is certainly not anywhere near 20 years old."
The man, who has been arrested, is quoted as saying: "I took her here to be examined - just do your job. Stop asking so many questions."
There is a history of Chinese men purchasing Vietnamese brides from across the country's southern border.
Police are continuing their investigation.
