SAN DIEGO -- Hopes of seeing Bai Yun deliver a second panda cub remained dim Thursday -- two days after the birth of the first of the twins she was carrying -- but she is showing no sign of distress, zoo officials said.

Between midnight and 1 a.m., members of the San Diego Zoo Panda Watch team observing Bai Yun in a nursing den through closed-circuit television saw motion representing bouts of contractions, said zoo spokesman Paul Garcia.

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But no birth immediately followed, and the zoo's panda experts continue to expect that the second cub will be stillborn, he said.

Nonetheless, the Panda Watch team members are "very pleased" because Bai Yun is taking excellent care of the cub born Tuesday, Garcia said.

"Nursing is going well," he said.

While disappointed at the absence of a second life birth, panda experts monitoring Bai Yun feel that her health is not an immediate source of concern, Garcia said.

"They feel confident that she's not at risk," he said.

Panda Watch team leader Doug Lindburg said Wednesday that there is a strong possibility that the second cub would be stillborn, but the monitors will let nature take its course -- a decision that remained in effect Thursday.

"Because of the high-risk situation," veterinarian Patrick Morris said, "it's not a very savory option to intervene as one would in other situations."

Bai Yun delivered the first of the two cubs at 1:15 p.m. Tuesday. According to the zoo, it is common for pandas to deliver twins, and for the births to occur several hours apart. But a second birth more than 24 hours after the first would be highly unusual.

The sex of the newborn, which weighs about 4 ounces, is unknown and probably won't be determined for about a week.

The birth brought to four the number of pandas now at the zoo. In addition to Bai Yun and her new cub, there is 4-year-old Hua Mei, and the male panda Gao Gao.