Bogota - Human traffickers in Mexico often hold the children of women sold into the sex trade hostage in order to force the mothers to keep working, campaigners said.

Traffickers prey on young and impoverished mothers, sometimes posing as a victims' boyfriend, and even marrying and having children with her, according to experts.

When women forced into prostitution do not earn their weekly quota, disobey rules or try to escape, traffickers threaten to keep or harm their children, they said.

"A way that traffickers coerce their victims in Mexico is by getting them pregnant, and then removing the child from their care," said Rosi Orozco, who heads the Commission United Against Human Trafficking, a Mexican non-governmental organisation.

"She is then threatened continually that she will never see her child again if she doesn't do as she is told," said Orozco, a former congresswoman who spearheaded Mexico's anti-trafficking law in 2012.

In the past five years, about 400 victims of sex trafficking have suffered this way, according to Teresa Ulloa, Latin America regional director for the Coalition Against Trafficking in Women.

Victims are commonly women and girls aged 15 to 25 living in impoverished areas across Mexico, Ulloa told the Thomson Reuters Foundation.

Women are lured with false offers of work as dancers or models in cities in Mexico and the United States only to be forced into prostitution on the streets and in brothels, she said.

In parts of the central Mexican states of Puebla and Tlaxcala, sex trafficking is usually a family-run business, and the children of victims become property of local gangs, according to Ulloa.

"There's, like, the godfather, the head of the family, then the whole family below him plays a role, some in looking after the children," Ulloa said.

"But they don't look after the children."

Children, including babies, rescued in anti-trafficking police operations have been found in "appalling, overcrowded" conditions, and some were tied up, Ulloa said.

"There were situations where children aged three were having to change the nappies of babies, and they had red, horrible rashes," she said.

Over the past decade, federal authorities have investigated 169 cases of human trafficking in Puebla and Tlaxcala, including forced prostitution, official figures show.

But survivors are unlikely to testify in cases against the perpetrators unless authorities are able to first rescue their children, Ulloa said.

"Women won't talk if they know their children are still in the hands of the traffickers," she said.

In Tlaxcala, sex trafficking is often a crime passed down through generations and regarded as a "cultural tradition", according to Jorge Sosa, a lawyer and anti-trafficking campaigner in Puebla.

"It's accepted in families that children prepare and aspire to become pimps," he said.

"Children even feel proud of saying that when they grow up they want to be pimps."

Globally, nearly five million people are forced into sexual exploitation, according to the International Labour Organisation.

Thomson Reuters Foundation