Edinburgh Zoo warned Monday that its female giant panda, Tian Tian, may have lost her cub.
Hopes were high that Tian Tian, or Sweetie, would give birth at the end of August following artificial insemination in April.
But the zoo's panda expert said Tian Tian is "now past her due date," and hormone tests showed that "something may be amiss."
"The evidence suggests that this may be bad news," said Iain Valentine at the Royal Zoological Society of Scotland. He added, however, that there is still a chance the panda will give birth, and experts are monitoring her closely.
Many thought it could be third time lucky for the panda, which failed to mate with male companion Yang Guang, or Sunshine, despite encouragement from zoo keepers. She became pregnant last year, also after artificial insemination, but appeared to have suffered a miscarriage late term.
Giant pandas have difficulty breeding and their pregnancies are notoriously difficult to follow. Their fetuses are tiny and hard to detect, and the animals also experience "pseudo-pregnancies" during which behaviour and hormonal changes indicate they are pregnant even when they are not.
But scientists believed that Tian Tian was indeed pregnant and was likely to carry to full term, Valentine said.
Tian Tian and Yang Guang, both aged 11, arrived from China in 2011. They are the only pandas in Britain.