Pandas frequently deliver twins several hours apart, but a gap of more than 24 hours in the births is unusual and could signal bad news.

"This is not the normal course of events for the species," said Don Lindburg, head of a team of doctors at San Diego Zoo where Bai Yun, a female panda on long-term loan from, China gave birth to her first cub Tuesday.

"I cannot tell you when this will arrive at a final solution. We simply are letting nature take its course," said Lindburg.

"We assume that the foetus 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," Lindburg said.

The arrival of the first twin cub boosted the zoo's population of the rare beasts to four.

The sex of the still-unnamed young panda is unknown and probably will be for about a week, officials said.

Bai Yun came to San Diego in 1996 with a male called Shi Shi, but the match turned out not to be made in heaven as the pair only managed one baby in six years.

Shi Shi was returned to China last year and replaced in January by 11-year-old Gao Gao as part of a drive to increase the world's dwindling giant panda population.

But ironically, it has emerged that Shi Shi might have belatedly fathered Bai Yun's twins through artificial insemination, zoo officials announced earlier this month.

Bai Yun's last cub, Hua Mei, a female born August 21, 1999, is the only surviving giant panda born in the United States.

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

One reason giant pandas are among the most endangered species in the world is their lack of interest in sex, which experts have sought to remedy with everything from Viagra to porn videos.

Only about 1,000 giant pandas are believed to be left in the wild.

TERRA.WIRE