A giant panda at the San Diego Zoo is a proud new mama. After 2 1/2 hours in labor, Bai Yun gave birth to her fourth cub.

Officials found out she was pregnant July 18 after they detected a heartbeat and saw the fetus on an ultrasound. The panda's sex is unknown and the head of the zoo's panda program says all they've seen so far is the tail.

The new baby is about the size of a stick of butter and zookeepers are keeping a watch on a closed-circuit camera for a glimpse of the cub. The cub's presumed father, Gao Gao, is also the father of two of Bai Yun's previous cubs. Notoriously hard to mate in captivity, fewer than 180 giant pandas live in captivity.