LONDON (AP) — Edinburgh Zoo is hoping for the pitter-patter of tiny paws, saying Friday that there are signs that one of its giant pandas may be pregnant.

The zoo said that nesting behavior and changes in protein and hormone levels suggest Tian Tian could be expecting a cub — or experiencing a phantom pregnancy, not uncommon in pandas.

The Scottish zoo said the signs "give us cause for encouragement," although Tian Tian has not cooperated with an ultrasound that could yield more information.

"Confirming a female panda’s pregnancy is never straightforward and we would encourage people to try not to get too excited just yet," said Ian Valentine, director of the zoo’s panda project. But, he said, "the overall picture is looking quite good."

If pregnant, Tian Tian should give birth in late August or early September. Panda pregnancies typically last about five months and result in one or two pink, hairless cubs.

Giant pandas have difficulty breeding, with females fertile for only two or three days a year. No giant panda cubs have ever been born in Britain.

The zoo artificially inseminated Tian Tian, or Sweetie, after she was reluctant to mate with male companion Yang Guang, or Sunshine. Keepers had tried to encourage the pair by building a "love tunnel" between their enclosures, but to no avail.

The two animals arrived from China in 2011 on a 10-year loan, and are the only pandas in Britain.