SAN DIEGO -- The San Diego Zoo announced Monday that Bai Yun, a giant panda on loan from China, is pregnant with twins, 10News reported.

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See Ultrasound Images Of Panda Twins

In March, she mated with new resident Gao Gao and was also artificially inseminated the following day with sperm from the other resident panda, Shi Shi, who has since returned to China.

Female giant pandas only go into estrus for 2 to 3 days a year, so the window of opportunity for successful mating is very limited.

The pregnancy was discovered Sunday via ultrasound, zoo officials said.

The twins are due sometime this month, though there is no specific projected date, according to 10News.

"This allows us to add more and more information to the body of scientific knowledge about panda reproduction. This is a big step and a real advantage," zoo veterinarian Pat Morris said.

The pandas are at the zoo as part of a 12-year research loan. Researchers want to learn more about the mating habits of the solitary animals.

Their low reproductive rate is one of the reasons the giant panda is endangered, with only about 1,000 living now. Other factors are habitat destruction, bamboo shortages and being caught in traps meant for other animals.