Zookeepers hope giant panda Mei Xiang could soon deliver a panda cub at the Smithsonian National Zoo. It's rare for panda cubs to be born in captivity. In the U.S., fewer than a dozen giant panda cubs have been born in zoos.

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One of them was actually Mei Xiangs previous cub, born in 2005. The panda was named Tai Shan and is now in a China breeding facility.

“It was monumental when he was born,” said the Zoo’s Chief Veterinarian Dr. Suzan Murray.

“He was really strong. We’re hoping for a repeat performance... If anyone can do it she can.”

Using sperm from companion Tian Tian, the 12-year-old panda has been artificially inseminated seven times, but none of those attempts were successful.

Mei Xiang is kept away from visitors for now. She’s on 24-hour watch, and zookeepers see via cameras that she's building a nest. Tests show elevated hormone levels consistent with pregnancy.

All giant pandas are on loan from China. If Mei Xiang has a cub, it would be able to stay at the Smithsonian Zoo for a couple of years. Then the cub would be sent to China.