CHENGDU -- An 18-year-old captive-bred panda, considered old among pandas eligible for pregnancy, gave her sixth birth in southwest China on Saturday.

Qizhen's water broke at around 6 a.m. The cub, the tenth by the mother, weighed 168 grams at birth. Both stay very healthy, said Wu Kongju, a panda expert with the Chengdu Giant Panda Breeding Center in Sichuan Province.

Giant pandas have extremely low birth rates as a couple has a very narrow window once a year to conceive. In general, a female panda gives birth every two years, to one or two cubs at a time.

They also stay fertile for only 12 years, mostly between 8 to 20.

Wu said Qizhen gave birth sixth times over the past 11 years. She can still have one more birth just before she turns 20.

"It will be very rare for a panda to have seven births in her lifetime," Wu said.