SAN DIEGO -- An around-the-clock birth watch has been established for a giant panda pregnant with twins, officials with the San Diego Zoo said Thursday.

SURVEY Bai Yun is pregnant with twins and the father could be either Gao Gao, Shi Shi or both. Who's the daddy? Gao Gao Shi Shi Both Results  |  Disclaimer

The watch by animal care staff and researchers was started after an ultrasound test on Bai Yun detected a rapid increase in a fetal growth rate inside her womb, said Don Lindburg, leader of the zoo's Giant Panda Conservation and Research Team.

"Although we are not certain when Bai Yun will go into labor, we felt it is a wise decision to begin monitoring her pregnancy 24 hours a day," he said.

The twins -- pandas usually have twins, but choose only to raise one of them -- were discovered in Bai Yun during an ultrasound test Aug. 3.

She mated with Gao Gao in late March, then was artificially inseminated with sperm from Shi Shi, who is now back in China.

There is no due date for the twins, but Lindburg said recently that the births could occur by the end of this month.

Zoo officials will need to be on hand to retrieve the newborn panda that the mother ignores.