Mei Xiang's urine samples have been showing a rise in progesterone. Her keepers think this is a promising sign.

The hormone spike means the female giant panda could be pregnant with a cub — but it might just be a red herring.

Giant pandas tend to undergo a "pseudopregnancy" that mimics the real thing when they don't conceive. Their hormone levels surge, they build nests, they lose their appetites, and they sleep more than usual, though when they're awake, they can sometimes seem restless.

Mei Xiang has been less interested in her keepers, won't finish her biscuits and "continues to be very sensitive to noise," according to a recent update from the Smithsonian's National Zoo in Washington, D.C., where the 15-year-old panda lives. The bear was even spotted cradling one of her toys, her keepers said.

This behavior and the hormone surge indicate that Mei Xiang could be nearing the end of a pseudopregnancy or could give birth to a cub, according to the zoo. [Baby Panda Pics: See A Cub Growing Up]

Mei Xiang's situation is not unique this time of year. Female pandas are only fertile for a few days, usually in the spring. Even if they do naturally mate or receive an artificial insemination during this brief window, it's almost impossible for zookeepers to detect true pregnancy in captive pandas until a tiny cub materializes in late summer.

Across the pond, in Scotland's Edinburgh Zoo, panda keepers are playing a similar waiting game with 9-year-old Tian Tian, Chinese for "sweetie."

"We cannot tell definitively at this stage if Tian Tian is pregnant or not, although we're seeing results that give us cause for encouragement," Iain Valentine, who is in charge of giant pandas for the Royal Zoological Society of Scotland, said in a statement.

"Tian Tian still may be experiencing a pseudopregnancy, so it is important to remember that and there is still a need to just watch and wait whilst continuing to monitor her hormone levels," Valentine added. "Confirming a female panda's pregnancy is never straightforward, and we would encourage people to try not to get too excited just yet — I know it's easier said than done though!"

Tian Tian had trouble mating naturally with her partner, so in April, the panda was inseminated with a mixture of fresh semen from her male companion Yang Guang and frozen semen from Bao Bao, a panda that died last year at the Berlin Zoo. Mei Xiang likewise was artificially inseminated twice in March after natural breeding attempts with the National Zoo's male panda failed.

These arduous captive breeding efforts are part of conservationists' attempts to bolster the endangered species' numbers and genetic diversity. There are just 1,600 pandas estimated to be left in the wild in China — in mountainous, often fragmented, forest habitats. All pandas in captivity technically belong to China, which loans the bears to foreign institutions.

These efforts do pay off. Already in this year's panda baby season, the United States has welcomed two cubs — a set of male panda twins born at Zoo Atlanta.

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This story was originally written for LiveScience and has been republished with permission here. Copyright 2013 LiveScience, a TechMediaNetwork company.