LONDON (AP) — The only female giant panda in Britain is believed to be pregnant, Edinburgh Zoo said Thursday.

The zoo said Tian Tian "is being closely monitored," but it's unclear when she will give birth. The zoo said "it's hard to predict precisely and the panda breeding season can last until late September."

Tian Tian and male panda Yang Guang, who are both 14, arrived in Edinburgh on a decade-long loan from China in 2011 and are the only giant pandas in Britain.

Tian Tian, or Sweetie, has been pregnant several times before in the U.K. but has never given birth. She had twin cubs in China in 2009.

Giant pandas have difficulty breeding and their pregnancies are notoriously difficult to follow. Their fetuses are tiny and hard to detect, and the animals also experience "pseudo-pregnancies" during which behavior and hormonal changes indicate they are pregnant even when they are not.

The zoo's statement came after the Edinburgh Evening News used the Freedom of Information Act to obtain correspondence between the zoo and Scottish government disclosing that Tian Tian had been artificially inseminated and could give birth as early as this week.

In a July 25 email, Iain Valentine, panda director at the Royal Zoological Society of Scotland, said he thought Tian Tian was about 30 days away from giving birth.