The researches were surprised by a panda who gave birth to twins in southwest China.
The first born was male and sex of the second born was unknown because the mother would not let it out of her grip after the Thursday birth, the official Xinhua News Agency reported.
"The mother and her babies are all well," Zhang Zhihe, director of the Chengdu Giant Panda Breeding and Research Center in Sichuan Province, was quoted as saying.
Female pandas normally become sexually mature between 4 and 5 years old. They can get pregnant once a year and usually give birth to one or two cubs at a time.
More than 20 pandas were born in captivity in China last year, the report said.
The panda is one of the world's rarest animals, with about 1,590 living in the wild in China, mostly in Sichuan and the western province of Shaanxi. Another 180 have been bred in captivity.