Bai Yun, the adult female panda living at California's San Diego Zoo, is pregnant again, the zoo announced Wednesday.

An ultrasound has confirmed that the 17-year-old panda is carrying two fetuses, although one of the twins may not be viable, said zoo officials.

Bai Yun, who came to the zoo in 1996 as part of a scientific exchange with China, has already given birth to four healthy cubs.

Her first and second cubs, Hua Mei and Mei Sheng, are currently in China under a breeding program, while other two offspring, Su Lin and Zhen Zhen, are still at the zoo.

Gao Gao, the Zoo's another adult panda, is the father. Zoo veterinarians said Bai Yun and Gao Gao mated in April and they expect the birth to occur in August.

Veterinarians said they were unable to tell if Bai Yun was pregnant until the ultrasound was taken. Despite years of research, much about the panda reproductive system remains a mystery.

In Bai Yun's two previous pregnancies, she had two fetuses each time but only one survived.

Source: Xinhua