Twin giant pandas each gave birth to twin cubs this week as the number of pandas born in captivity this year in China rose to six, state media reported Tuesday.

The sisters, Qi Zhen and Qi Yuan, delivered their babies on Sunday and Monday at the Chengdu Giant Panda Reproduction and Research Center in Chengdu, the capital of southwest China's Sichuan province, where they live, the official Xinhua News Agency said.

A 7.7-ounce panda cub was born on Monday at the Wolong Giant Panda Protection and Research Center, elsewhere in Sichuan, and it became the heaviest cub in the history of China's artificial breeding program, according to the report.

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Its mother, Zhang Ka, also set a record for the longest delivery after about 34 hours in labor, Xinhua said.

The first panda born in captivity this year was delivered at the Wolong center on June 22.

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.

The panda is one of the world's rarest animals, with just 1,596 left in the wild, according to a 2002 government census.