ENUGU, Nigeria (AFP) – Neighbors were suspicious of the daytime silence at the maternity clinic that came to life only after nightfall, though never suspected its disquieting secret -- it was breeding babies for sale.

But recent police raids have revealed an alleged network of such clinics, dubbed baby "farms" or "factories" in the local press, forcing a new look at the scope of people trafficking in Nigeria.

At the hospital in Enugu, a large city in Nigeria's southeast, 20 teenage girls were rescued in May in a police swoop on what was believed to be one of the largest infant trafficking rings in the west African country.

The two-storey building on a dusty street in Enugu's teeming Uwani district now stands deserted, shutters down.

Neighbours had long found something bizarre about the establishment, where there was virtually no activity during the day, they told AFP.

The doctor in charge, who is now on trial, reportedly lured teenagers with unwanted pregnancies by offering to help with abortion.

They would be locked up there until they gave birth, whereupon they would be forced to give up their babies for a token fee of around 20,000 naira (170 dollars, 135 euros).

The babies would then be sold to buyers for anything between 300,000 and 450,000 naira (2,500 and 3,800 dollars) each, according to a state agency fighting human trafficking in Nigeria, the National Agency for the Prohibition of Trafficking in Persons (NAPTIP).

But luck ran out for the gynaecologist, said to be in his 50s, when a woman to whom he had sold a day-old infant, was caught by Nigeria's Security and Civil Defence Service (NSCDS) while trying to smuggle the child to Lagos, the security agency said.

Statistics on the prevalence of baby breeding are hard to come by, but anti-trafficking campaigners say it is widespread and run by well-organised criminal syndicates.

"We believe the scope is much wider than we know," said Ijeoma Okoronkwo, head of NAPTIP.

"It has been happening over time, but we did not know. The first indication we had about this came in December 2006, when an NGO raised the alarm and told us babies were being exchanged for cash and that there were a number of hospitals involved," she told AFP.

The practice takes varying forms. One is where desperate teenagers with unplanned pregnancies, fearing ostracism by society, get lured to a clinic and are forced to turn over their babies.

The girls are so intimidated many can hardly relate their experience freely.

But one brave victim, an 18-year old, who asked not to be named for fear of reprisal, recounted her week-long ordeal when she was trapped inside one of the clinics days before it was raided by police.

"The moment I stepped in there, I was given an injection, I passed out and next thing I woke up and realised I had been raped," the girl, who was five months pregnant at the time of her ordeal, told AFP.

When she asked if she could telephone her family to let them know of her whereabouts, the doctor slapped her on the face.

She was shoved into a room where 19 other girls were kept; all had been through a similar experience. She said the doctor raped her again the following day. A week later police swooped on the clinic.

Another category of young women, driven by deep poverty, lease out their wombs and volunteer themselves, as regularly as is biologically possible, to produce babies for sale.

"When we raided the hospital, we found four women who had been staying at the clinic for up to three years, to breed babies," NSDCS boss for Enugu state commandant Desmond Agu told AFP.

The doctor, whom police named, "had been inviting boys to come and impregnate girls," said Agu.

This was just one of around a dozen centres -- masquerading as maternity clinics, foster homes, orphanages or shelters for homeless pregnant girls -- unearthed in recent months where babies were swapped for cash, said the NAPTITP boss.

Last month police swooped on a so-called foster home, not far from the Enugu police headquarters, where seven teenage pregnant girls and five workers were rounded up, residents said.

In 2005, a Lagos-based orphanage suspected of ties to child trafficking rings, was shut down. There, charred baby-bones were discovered on the rubbish tip, leading to suspicion the orphanage was involved in the peddling of human body parts, possibly for use in rituals or for organ harvesting.

In other cases observers say babies are purchased to be raised for child labour and sexual abuse or prostitution.

Trafficking in humans has become a lucrative trade.

Globally, it is estimated that billions of dollars exchange hands annually for payment of humans, according to the International Labour Organisation (ILO) and several UN agencies.

Witchcraft rituals also fuel baby trafficking, but experts say it is other motives that predominate, at least in this region of Nigeria.

Communities frown on children born out of wedlock and childlessness in marriage remains a curse for the woman.

"In the Igbo society, the price to remain childless is too high," said a clinical psychologist Peter Egbigbo.

"Childless people want to pay any amount for a child and doctors become rich overnight," he said, adding that those who are ready to adopt a baby would rather hide the fact that it is not their biological child.

Exchanging babies for cash is widespread in the region and in many cases locals do not see anything wrong in so doing.

"Many people don't even know what they are doing is criminal. They just think it's adoption -- you walk into a clinic, pay a fee and you have a baby," said Okoronkwo.

Buying or selling of babies is illegal in Nigeria and can carry a 14-year jail term.

It is estimated that globally hundreds of thousands of people are trafficked annually. UNICEF, the United Nations Children's Fund, estimates that at least 10 children are sold daily across Nigeria, where human trafficking is ranked the third most common crime after economic fraud and drug trafficking, according to UNESCO.

"There is so much profit in this business. There is so much to be made in trafficking and that is why it is thriving.