SAN DIEGO -- Zoo officials announced Wednesday that giant panda Bai Yun and her first cub are doing very well. However, the second cub is yet to be born and "a favorable outcome is increasingly unlikely at this point."

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Officials said that they will not intervene unless there is a clear medical emergency. They said "nature will take its course."

Bai Yun showed signs of contractions early Wednesday morning but those signs have since diminished.

"We assume that the fetus is still in her body. We assume that she will eventually pass it. What we cannot tell you is if it will be alive or stillborn," panda team leader Don Lindburg said.

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Bai Yun gave birth to her first cub at 1:14 p.m. Tuesday. It weighed in at 4 ounces.

The sex of the first cub is unknown and probably will be for about a week.

Twin deliveries of pandas is fairly common, happening in about 50 percent of pregnancy cases, according to the zoo.

The mother will typically reject the second cub, but zoo officials will step in to take care of the new panda's sibling when and if it arrives.

The zoo's other pandas are the male Gao Gao and 3 1/2-year-old Hua Mei.