Police in Nigeria's largest city Lagos said Wednesday they had discovered another illegal maternity unit where captives were impregnated and forced to give birth to babies for sale.
Authorities discovered the compound in the Isolo district after officers found seven pregnant women and girls who had fled the centre.
The revelation came just a few days after police said they had rescued 19 expectant mothers from several similar facilities in another area of the sprawling city.
"We got a tip-off about seven pregnant women waiting at a bus stop and our men went there to pick them up," Lagos police spokesman Bala Elkana said.
"After interrogation, the women said they were (among) 20 women who had been made pregnant in the home and all of them had escaped on Monday night," he said.
He said only those seven, aged between 13 and 27, had been found and the remaining 13 must have fled to other locations.
Elkana described the Isolo home as "a detention centre where young women are made pregnant to have babies who are later sold".
He said the police were on the trail of the those behind "this inhuman and heinous crime."
So-called "baby factories" -- a key link in a grim trade of selling infants to childless couples -- are notorious in Nigeria, where most of the population of 200 million live in poverty.
The police said on Monday that they raided four buildings used as illegal maternity units in the city on September 19 and rescued 19 pregnant women and girls and four babies.
The police said a majority of the women were brought to Lagos with promises of domestic work before being abducted and forced to have children.
Baby boys are typically sold for 500,000 naira ($1,400) while girls fetch 300,000 naira, according to the police.