SAN DIEGO - A 13-year-old giant panda gave birth to a cub at San Diego Zoo, officials said Wednesday.

Bai Yun gave birth shortly before 10 p.m. Tuesday after being in labor for three hours. The birth - the second in less than a month at a U.S. zoo - was captured on closed-circuit television in the Giant Panda Research Station birthing den.

Zoo officials said the cub weighed 4 ounces, the size of a stick of butter. The gender will not be known for some time because zoo officials don't want to disrupt mother and child.

"The birth of a giant panda cub is definitely something to celebrate," said Yadira Galindo, the zoo's spokeswoman.

Breeding pandas in captivity is a particularly difficult process, in part because females are in heat for only a day or two per year, and many cubs do not survive infancy. It wasn't until 1999, when Bai Yun had a female cub, Hua Mei, that a panda born in the U.S. survived into adulthood. Bai Yun successfully gave birth to a second cub in 2003.

The panda born Tuesday joins a cub born July 9 at the National Zoo in Washington. Zoo officials there announced Tuesday that they had finally been able to examine the little creature while the panda's mother, 6-year-old Mei Xiang, left the den to eat, and determined it is male.

There are only about 1,600 giant pandas in captivity and in the wild, zoo officials said.