Cambodian women who were charged with human trafficking for serving as surrogate mothers have been provisionally released from detention after agreeing to keep their babies rather than giving them up as originally planned.

Key points: Some Cambodian woman have been offered as much as $13,800 to become surrogates

Some Cambodian woman have been offered as much as $13,800 to become surrogates Surrogacy services can cost about $207,000 in countries like Australia or the US

Surrogacy services can cost about $207,000 in countries like Australia or the US The surrogacy business boomed in Cambodia after a crackdown in Thailand but has now shifted to Laos

The 32 women, who were charged in July with violating surrogacy and human trafficking laws, were released on bail in three groups, the last 17 on Wednesday (local time), Bun Samkhan, a spokeswoman for the National Committee for Counter Trafficking, said.

A senior police officer who works at the same agency said the women were released on humanitarian grounds.

He said they had committed crimes but their babies were innocent and for that reason the committee requested the court free them.

They had been held at a police hospital.

The identifies of the fathers have not been released.

Under Cambodian law, acting as an intermediary between an adoptive parent and a pregnant woman carries a penalty of one to six months in prison.

The human trafficking offence is punishable by seven to 15 years' imprisonment.

A Chinese man and four Cambodian women accused of managing the surrogacy business were charged with the same offences.

Sorry, this video has expired Sam Everingham, global director of Families Through Surrogacy, speaks to The World about the release of Cambodian Surrogate Mothers.

Ms Samkhan said her committee requested the release of the surrogate mothers so they could take care of their babies, with the condition they keep the children.

"We have told them clearly that these babies belong to you, so you have to take care of them until they grow up, and not sell them," Ms Samkhan said.

"And they agreed."

Cambodian women who have served as surrogates have said they were offered as much as $US10,000 ($13,822).

The average annual household income in Cambodia is about $2,059, according to the International Monetary Fund.

It was not known how much the women in this group were paid.

Surrogacy services shift countries amid crackdowns

Cambodia passed a law specifically targeting surrogacy in 2016 as the country was becoming a popular destination for foreign would-be parents seeking women to give birth to their children.

Developing countries are popular for surrogacy because costs are much lower than in countries like Australia and the US, where surrogate services can cost around $207,000.

Tammy Davis-Charles is questioned by officials in Phnom Penh. ( Supplied: Cambodian National Police )

The surrogacy business boomed in Cambodia after it was put under tight restrictions in neighbouring Thailand.

There were similar moves to curb the practice in India and Nepal.

After Cambodia's crackdown, the trade shifted to Laos.

In July last year a Cambodian court sentenced Australian nurse Tammy Davis-Charles and two Cambodian associates to 18 months in prison for providing commercial surrogacy services.

Davis-Charles was quietly freed earlier this year.

ABC/AP