The sex of the cub will not be known for some time, according to zoo officials, who observed the 1:30 p.m. birth and 2.5-hour labor through a closed circuit camera in the panda's birthing den.The Zoo's "Panda Cam" will provide a sneak peek online into the den within the next 24 hours at the San Diego Zoo's Web site.Bai Yun, 16, who was brought here from China, is the mother of all four pandas born at the zoo.It was not disclosed who the father was, but zoo officials reported in April that Bai Yun and Gao Gao had mated several times.Bai Yun has remained in an off-exhibit habitat for the past two weeks where she has slowly been showing signs of a pregnancy, including changes in her hormone levels, nest building and loss of appetite, zoo officials said.Most recently Bai Yun's mammary glands have swollen and she is almost exclusively spending her time indoors, according to the zoo.Bai Yun's first cub, Hua Mei, was born at the San Diego Zoo in 1999, becoming the first giant panda that was born in the United States to survive. Hua Mei was later sent to China where she gave birth to three sets of twins.Bai Yun's second cub, a male named Mei Sheng, was born in 2003 and is slated to be sent to China in October, zoo officials said. Her third cub, Su Lin, was born in 2005. Both Mei Sheng and Su Lin were sired by Gao Gao.The San Diego Zoo contributes more than $1 million each year to China for the program."Pandas were once faced with worldwide reproductive failure in breeding facilities, but through a decade of improved science and husbandry we and others are now able to routinely coax these rare and beautiful animals to do what comes naturally, resulting in a healthier population," said Ron Swaisgood, co-head of the San Diego Zoo's Giant Panda Conservation Unit.