San Diegp, Aug. 4 (AP): A giant panda at the San Diego Zoo has given birth to her fourth offspring

The cub was born on Friday, but its gender was not immediately known. The mother, 16-year-old Bai Yun, had been placed on 24-hour watch after officials detected a fetus and fetal heartbeat through ultrasound images on July 18.

``All we've seen so far is a leg and a tail,'' said Dr. Ron Swaisgood, co-head of the zoo's panda program.

The panda showed signs of going into labor early Friday, getting restless and frequently shifting position in a private den, Swaisgood said. When the cub finally came, Bai Yun quickly scooped it up and clutched it to her breast.

The cub hardly cried at all _ evidence of its mother's deftness.

``Usually the mother will bobble the cub or her paw will slip and the cub will cry until it's repositioned,'' said Swaisgood. ``But (Bai Yun) was keeping that cub so content it didn't cry at all. It made a few squawks and that was it.''

The newborn panda is about the size of a stick of butter, or one-thousandth the size of its mother, Swaisgood said.

Zookeepers won't get near the cub for at least a few days. They hope to get a clear view of the newborn on a closed-circuit camera, but typically a mother panda will hunch protectively over the cub to obscure it.

``This is a delicate period for the cub, but Bai Yun is an experienced mother,'' the zoo said in a press release.

In August 1999, Bai Yun gave birth to Hua Mei, who has, in turn, given birth to three sets of twins since moving to China in 2004, including a pair in July.

Mei Sheng, a male panda born in August 2003, will leave San Diego in October to join China's panda breeding program. Bai Yun's third cub, Su Lin, was born in August 2005.

Bai Yun, which translates as White Cloud, appears to have been impregnated about eight weeks ago by Gao Gao, also the father of her two previous cubs.

Only about 1,600 giant pandas remain in the wild, and fewer than 180 live in captivity.