Samantha Orobator, 20, was facing death by firing squad for drug trafficking, said Clare Algar, the executive director of Reprieve, a London-based human rights group.
She was arrested August 5, Khenthong has said.
Orobator was alleged to have been carrying just over half a kilogram (about 1lb) of heroin, Reprieve lawyer Anna Morris told CNN by phone from Vientiane, the Laotian capital. Those found guilty of carrying that amount normally face the death penalty, she said.
Reprieve has said Orobator became pregnant in prison, possibly as a result of rape, and that she is due to give birth in September. That would mean Orobator became pregnant in January.