On the wall of a toilet stall in an Internet cafe on Meichuan Road in Putuo District in Shanghai hangs the torn remnant of a leaflet offering ultrasound to pregnant women who want to know the sex of their unborn.

"Half-hour scan to know the baby's gender" and "painless abortion" are the only words remaining on the anonymous ad, which gives a mobile number but no address. No doubt, the leaflet was ripped from the wall by police.

In China, where the one-child policy often collides with the traditional preference for a son in the family, ultrasound to determine the gender of a fetus is banned except for medical reasons.

That has created an underground in illegal ultrasound services.

Unlicensed doctors - or people who pretend to be doctors - take pregnant women into shadowy clinics to offer them ultrasound scans. Often these charlatans use veterinary ultrasound devices developed for pigs and cows.

After observing a vague shadow on the screen and sometimes little more than guessing at what they are seeing, these "doctors" reveal a baby's gender to the mother. If she is not pleased by the news, they usually offer a cheap abortion.

Municipal health authorities are well aware of these illegal practices, but stamping them out is proving difficult.

"Our staff must wear uniforms when investigating these illegal clinics for law enforcement, and that just causes these people to flee at the sight of us," said Hu Xuewei, an official with the Shanghai Health Supervision Agency. "We want local people to give us more tips to help us stop such illegal medical practitioners and protect public health and safety."

The unlicensed doctors are cunning. They don't reveal themselves before carefully sizing up potential "patients" from a safe distance.

"I have scanned hundreds of times and the results are all correct," a "doctor" told a reporter who called the number on a leaflet. "It's 200 yuan (US$31.39) for the scan if the result is a boy and 100 yuan if it's a girl. Call me again when you are standing at the corner with your pregnant wife."

The reporter went to the corner of Meichuan and Wanzhen Road and called the "doctor" again.

"I can see you there," the voice on the phone said. "But where is the woman? Sorry, but no business with you."

And then he hung up.

By talking to market vendors in the area, the reporter was able to track down a relative of the ultrasound "doctor." The man, who refused to be identified, told Shanghai Daily that his brother uses a portable device and takes patients to rented rooms, shrouded park areas or even toilets for a fast scan. Those wanting an abortion are taken to a clinic, but neither the man nor any area merchants said they knew its location.

"It's fine if you take the scan only for reference," the man said. "But I would think twice about having an abortion with my brother. People should go to hospitals for that."

Not much brotherly trust there.

Ultrasound devices for veterinary use cost between 2,000 yuan and 8,000 yuan, and can be bought online without providing any medical credentials.

It's sometimes hard to fathom why some people would be so obsessed with baby gender that they would place themselves in the hands of charlatans.

"We had a daughter but we still wanted to have a son," said an Anhui Province vendor in the market surnamed Lin.

Under China's family-planning laws, urban couples are allowed to have a second child only under restricted conditions, such as both spouses coming from one-child families or a first child inflicted with a non-hereditary disease.

Shanghai has managed to prevent the births of about 7 million people since being one of the first batches of cities adopting the tough policy in the late 1970s, according to Shanghai Population and Family Planning Commission.

The natural growth rate of people with registered residency has been kept on a negative curve since 1993.

Now the city is cracking down on the ultrasound clinics. Last year, Shanghai closed 716 illegal clinics and apprehended 698 unlicensed doctors offering illegal ultrasound scans and abortions.

In some rural areas, couples are allowed a second child if their firstborn is a girl, since sons are considered crucial to labor in agrarian societies.

"We couldn't afford to have more than two children, so we wanted to make sure that the second child was a boy," said Lin.

He took his wife to a clinic deep into a narrow lane. The "doctor" said the scan showed a second daughter, but that he had abandoned follow-up abortions after a woman died on his operating table.