LONDON — Edinburgh Zoo says the only female giant panda in Britain is believed to be pregnant.

The zoo said Thursday that Tian Tian "is being closely monitored," but it can't say when she will give birth. The zoo says "it's hard to predict precisely and the panda breeding season can last until late September."

Tian Tian and male panda Yang Guang arrived in Edinburgh from on a decade-long loan China in 2011 and are Britain's only pandas.

Tian Tian, or Sweetie, has been pregnant several times before in the U.K. but has never given birth.

Giant pandas have difficulty breeding and their pregnancies are notoriously difficult to follow. The mammals' fetuses don't start to develop until the final weeks of gestation.