LONDON, England (CNN) — A pregnant British woman held on drug-smuggling charges in Laos will go on trial Wednesday, the British Foreign Office said, citing the government of the south-east Asian country.

Samantha Orobator, 20, was jailed last August at the airport in the Lao capital Vientiane and charged with carrying about half a kilogram of heroin. She is more than five months pregnant, and legal aid charity Reprieve — which has worked on her case — is urging that her case be resolved and she be allowed to return to England before she enters the third trimester of pregnancy on June 6.

London and Vientiane signed a prisoner transfer agreement last month, the Foreign Office said, but refused to disclose its details.

Orobator became pregnant in prison under circumstances which remain unclear.

She told her mother she was not raped in prison and that the father is not a Lao prison official, Jane Orobator said May 19 after visiting her daughter.

A newspaper run by the Lao government reported Tuesday that Orobator said she impregnated herself with the sperm of another prisoner being held in the same jail.

A Reprieve spokeswoman said the charity did not know if the newspaper report was true.

“We haven’t seen her to confirm this, but are keen that this issue — which is irrelevant to the case — does not delay or affect the trial,” Katherine O’Shea told CNN via e-mail. “Our first priority is the health of mother and baby, which means swift trial and return to the U.K.”

The British Foreign Office refused to comment on the newspaper report, saying it would not be able to talk about the case until the trial ends.

A spokeswoman said the British government did not know how long the trial would last. She declined to be named, in line with policy.