Giant panda Tian Tian is believed to be pregnant and may give birth by the end of the month.
Keepers at Edinburgh Zoo have been monitoring Tian Tian since she was artifically inseminated on April 13.
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Iain Valentine, director of pandas at the zoo, said: "The latest scientific data suggests Tian Tian the giant panda is now pregnant and that implantation has taken place; therefore, she may give birth at the end of the month.
"This is all very new and complex science and we still have a bit of time to go yet, as like last year, the late loss of a cub remains entirely possible."
Tian Tian (Sweetie) and 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.
Experts have been analysing the animal's hormone and protein levels but said it will not be known for certain if she is pregnant until she gives birth.
Mr Valentine, from the Royal Zoological Society of Scotland, said the latest scientific tests will give a strong indication but are "too new" to be definitive.
He said: "Monitoring a female giant panda's behaviour - for example, if she is sleeping a lot, eating more or spending time in her cubbing den - is not an indicator of if she is pregnant or otherwise, as giant pandas experience pseudo pregnancies and she will show 'pregnant' type behaviour whether she is pregnant or not.
"Two of our Chinese colleagues are due to travel to Scotland in mid-August and we continue to monitor and wait."
Last year's attempt was the first artificial insemination procedure to take place on a giant panda in the UK.
Tian Tian did become pregnant but it is believed she reabsorbed the foetus.
She was again inseminated in April after she and Yang Guang failed to mate naturally.
Last month the zoo confirmed that Tian Tian had conceived but was not yet technically pregnant.
It said the panda was in "great health", of a good weight and eating well.
The panda gestation period is typically around five months and one or two cubs will be born.
They enter the world blind, hairless and unable to move - making them entirely dependent on their mother for survival for their first weeks.
The zoo said the panda breeding programme can play an important role in conservation.
Visitors flocked to the giant panda enclosure at the zoo today despite Tian Tian being off show during her pregnancy.
Families queued up to get a glimpse of her partner Yang Guang, but there was no sign of him leaving his private shelter in the wind and drizzle.
Nick Cook, 45, visited the zoo with his wife and four children while on holiday from Christchurch, New Zealand.
Mr Cook, originally from Hamilton, South Lanarkshire, said: "Pandas are endangered and they are also very cute so it is exciting news.
"Everybody loves to see them."
Angela Murray, 49, travelled from Lancaster with her son Thomas, six, and daughter Bethany, 16.
She said: "We've come on holiday especially to see the pandas so we're very excited to hear the female is expecting.
"If it does turn out there is a panda baby, we'll come back next year to see it."
The Folliard family, from Co Mayo in Ireland, also came on holiday to Edinburgh to see the pandas.
Father-of-two Liam Folliard, 49, said: "My son Jack is mad about pandas so we booked at Christmas time.
"We won't see Tian Tian now but we were hoping to see Yang Guang. We went for something to eat and I think we missed our chance."
Cathriona Folliard, 44, the mother of 11-year-old Jack and Ellie, nine, said: "I think next year we'll go and see the pandas in Madrid. There's better weather there."
Speaking at the zoo, Mr Valentine said: "It's good to be able to say to people, yes, our panda is pregnant, but we're also trying to say there's still a bit to go, it could go wrong.
"We have been here before but at this stage things are looking different and we are positive."
Here are some facts about panda cubs as keepers at Edinburgh Zoo announce they believe Tian Tian, the UK's only female giant panda, is pregnant.
- At birth, a cub is just 1,000th of its mother's weight - around 5.3oz (150g).
- Giant panda foetuses do not start to develop until the final weeks of gestation.
- Panda cubs are born pink and covered in short, sparse white hair, their eyes are tightly shut and they cry loudly and often.
- Their black patches start to appear at around one week old, followed by black hair on the patches a few weeks later.
- It is several weeks before they can crawl and cubs spend the first few weeks of life vocalising their needs to their mother, sleeping and suckling.
- Panda mothers lick their cubs often and do not leave the cub to eat bamboo until offspring are three to four weeks old. At this point the cubs can regulate their body temperatures and do not need constant body contact from the mother to keep warm.
- After a month, the cub looks much more like a miniature adult giant panda but with a longer tail. Its eyes open partly after 30 to 45 days, and fully open a week or two later.
- Panda cubs grow to up to 10 times their birth weight in the first five to six weeks.
- By 75 to 80 days, cubs can stand and walk a few steps; they also begin to teethe at this point and their eyesight and hearing improve.
- At four months, cubs are active, run about and climb on their mother's back to play.
- At five months, they trot behind their mothers and mimic them, eating bamboo and climbing trees.
- They start to eat solids at six months.
- A panda cub starts to get its permanent teeth at around one year old, when it will weigh around 50lb-60lb (23kg-27kg) and now only suckle once or twice a day.
- At the age of two, the cub would be returned to China, mimicking natural dispersal age in the wild.