The UK's only female giant panda Tian Tian could give birth soon after zookeepers revealed she is believed to be pregnant.
The surprise news came to light in correspondence between Edinburgh Zoo and the Scottish Government which suggested the due date could be Friday.
However, the zoo said the exact date was hard to predict.
A spokeswoman for Royal Zoological Society of Scotland Edinburgh Zoo said: "Giant panda breeding is a very complicated process but we believe that Tian Tian is pregnant.
"Although a specific date was suggested, like all babies it's hard to predict precisely and the panda breeding season can last until late September.
"Tian Tian is being closely monitored by our expert team and we will be the first to share any news as soon as we can."
It follows repeated attempts over several years to see if the female giant panda could produce a cub.
Panda reproduction is a notoriously tricky process, with females only ovulating once a year.
Artificial insemination was attempted for a fifth time after the zoo's male panda Yang Guang was unreceptive to natural mating.
Tian Tian (Sweetie) and Yang Guang (Sunshine) are the only giant pandas living in the UK.
The black and white bears arrived on loan from China in December 2011 and are due to remain at Edinburgh Zoo for a decade.
It was in 2008 that the zoo first announced it was in negotiations to bring a pair of giant pandas to Scotland.
The two were brought to the UK under a historic agreement between the UK and Chinese governments.
The pair, described as a gift from China, were the first giant pandas to reside in the UK for 17 years.
Their arrival marked the culmination of a five-year effort to bring them to Scotland, officials said.