Prized San Diego Zoo panda mom Bai Yun has given birth to her fifth cub, the zoo announced Wednesday.
The cub, born to 17-year-old Bai Yun, becomes just the 13th panda in the United States.
The zoo refused to release any more details about the birth before a news conference.
A baby panda weighs only about 4 ounces and is about as big as a stick of butter. The gender of the cub won't be known until veterinarians examine it further and it won't get a name for 100 days, according to Chinese tradition.
Mom and cub will lead private lives for the next four months or so, but they will appear on the zoo's live Panda Cam, which can be watched online.
The zoo announced just last week that Bai Yun was pregnant, based on ultrasound tests.
The father is longtime consort Gao Gao (Big Big), who has fathered three of Bai Yun's other cubs.
The number of cubs makes the pair one of the most reproductively successful panda couples ever in captivity.
Pandas are notoriously poor breeders - one reason their species is endangered - and females have only three days a year in which they can conceive. Only about 1,500 giant pandas remain in the wild, and around 250 live in captivity.
Bai Yun and Gao Gao come together only a couple of days a year. When Bai Yun enters her fertile periods, zookeepers make sure Gao Gao is there, sniffing her through a perforated gate zookeepers call a "howdy door" until her chirps and bleats indicate she's ready to get down to business.
Bai Yun gave birth to her first cub in 1999 through artificial insemination from her first arranged suitor, Shi Shi (Stone). Hua Mei (China/U.S.A.) was the first giant panda cub born in the United States after a decade of failed breeding attempts. She has had three sets of twins since returning to China in 2003.
Gao Gao, who was born in the wild, arrived in San Diego in 2003 after veterinarians gave up on Shi Shi, who turned out to be older and less virile than originally believed and was returned to China. Shi Shi died last year.
Some males never succeed at natural breeding, so artificial insemination has become common practice when breeding captive pandas.
