CHENGDU, China (CNN) -- Cheng Cheng, a 13-year-old panda at the Chengdu Giant Panda Breeding Base in Sichuan province, has given birth to twins.

It was the fourth time Cheng Cheng gave birth, and the breeding center's fourth and fifth births this year. Pandas are difficult to breed in captivity and, because of the decline in their natural habitat, only about 1,000 pandas are believed to be left in the wild.

The twins weighed five ounces (140 grams) and two ounces (58 grams). No names are reported yet.

Reuters contributed to this report.