Female giant panda Bai Yun is pictured in a video frame grab after giving birth to a cub at the San Diego Zoo July 29, 2012, in this handout photo provided to Reuters July 30, 2012. 20-year-old Bai Yun is one of the oldest giant pandas known to give birth to a cub, which are born without sight with an average weight of 4 ounces. This is her sixth cub, setting a record for the most cubs born at a breeding facility outside of China. [Photo/Agencies]

SAN DIEGO - The most prolific giant panda in captivity outside of China, Bai Yun, has proved her mettle yet again with the delivery of her sixth live cub at the San Diego Zoo - an unnamed baby panda that weighed about 4 ounces (113 grams) at birth.

Bai Yun, who gave birth on Sunday, was the first giant panda born in captivity in China, where giant pandas are an endangered species.    

The 20-year-old panda's pregnancy was considered high risk because of her age, according to zoo spokeswoman Christina Simmons. Bai Yun, whose name means White Cloud, is on long-term loan to the zoo from China.

Since 1999, Bai Yun has delivered three female and two male cubs, all but one fathered by her companion, Gao Gao. The sex of the new cub will not be known for several months, Simmons said.

The baby was born around 2:10 pm PDT (2110 GMT) Sunday, after about three hours of labor - longer than normal for Bai Yun, Simmons said.

"When they heard the cry from the baby, the veterinary staff was relieved for the mom and delighted for the baby," Simmons said. "It is about the size of a stick of butter and pandas are born hairless and pink."

Bai Yun and the new cub are in seclusion during what zookeepers call "a critical bonding period."  As long as everything appears to be going well, zoo veterinarians leave the mom and cub alone, Simmons said.  

"Panda moms and cubs go into their dens and the babies stay there for a long time," Simmons said. "We don't expect the baby to emerge before December."

The cub's father, Gao Gao, is not spending time with Bai Yun, Simmons said.

"Gao Gao is not particularly aware of what's going on," she explained. Pandas are normally solitary animals - they only associate with other adult pandas for breeding."

The birth of the cub brings San Diego Zoo panda count to four: the parents, a 3-year-old son named Yun Zi and the new cub. That's more than any zoo in the world outside China, Simmons said. Four of Bai Yun's other cubs were later sent on to China.

"We work very closely with the Wolong Giant Panda Research Center in China, and our good fortune has a lot to do with our intense collaboration with them," Simmons said.