The UK's only female giant panda has passed her due date and experts fear "bad news" regarding her pregnancy.
Experts at Edinburgh Zoo said Tian Tian should have gone into labour over the weekend but hormone tests suggested "something might be amiss".
However, the zoo said there is " still a chance" she could give birth to a live cub as her progesterone levels have not yet returned to base level.
She is still showing some of the behaviour typical of a pregnant panda, although scientific data from the urine analysis of her hormones is becoming more atypical, the zoo said.
The zoo announced two weeks ago that Tian Tian was thought to be pregnant and could give birth by the end of August and she has been closely monitored ever since.
She was artifically inseminated on April 13 after she and her intended partner Yang Guang failed to mate naturally.
Iain Valentine, director of giant pandas for the Royal Zoological Society of Scotland, said: "As you are all probably aware, giant panda Tian Tian is now past her due date and the evidence suggests that this may be bad news.
"She is still displaying some of the behaviours of a pregnant panda, but the scientific data from the urine analysis of her hormones is becoming more atypical. There is still a chance she will give birth to a live cub as her progesterone levels have not yet returned to base.
"I must stress, as there has been a lot in the news recently about pandas 'faking' it, that this was definitely not a pseudo or phantom pregnancy.
"The results of cutting-edge scientific analysis have shown that, across the entire pregnancy, Tian Tian had the profile of a pregnant panda likely to carry to full term. We are working with the very best panda experts in the world and they were all in agreement.
"The scientific data extracted by analysing hormones and proteins in her urine was a text book example of what we wanted to see. However, at the very end of last week there were one or two results from the hormone tests that were atypical and that was the first sign something might be amiss.
"Although it is still very new, the scientific data does suggest that the last point she should have gone into labour was over the weekend. Unfortunately this did not happen."
The zoo said its experts continue to monitor Tian Tian closely and will know more towards the end of the week as her cycle comes to an end.
Tian Tian (Sweetie) and male Yang Guang (Sunshine) are the first giant pandas to live in the UK for 17 years.
The animals, now both aged 10, arrived on loan from China in December 2011 and will remain at Edinburgh Zoo for a decade.
Tian Tian was successfully inseminated last year but lost her cub at late term.
The attempt was the first artificial insemination procedure to take place on a giant panda in the UK.
She was again inseminated in April and the zoo confirmed in July that she had conceived but was not yet technically pregnant.
Experts said it would not be known for certain if she was pregnant until she gave birth.
Edinburgh based animal protection charity OneKind has called on the Royal Zoological Society of Scotland, which owns the zoo, to abandon is attempts to produce a panda cub through artificial insemination.
The charity said it was sorry to hear the news about the pregnancy but that now is the time to leave the animals in peace.
OneKind Policy Director Libby Anderson said: "Unlike a human mother who makes the choice to undergo artificial insemination, Tian Tian has no say in whether she has these procedures.
"OneKind has always believed that it is misguided to attempt to breed more captive pandas when they will never return to the wild or improve protection for the wild population in their native habitat."