Panda mother Mei Xiang was artificially inseminated with sperm from Tian Tian, as well as a panda named Gao Gao at the San Diego Zoo. It takes time to determine a cub's gender, and a DNA sample was collected to determine the paternity of the cub born Aug. 23.
The second stillborn cub delivered a day later was also female and also sired by Tian Tian, officials said. The cubs were fraternal twins.
Keepers performed a den check Thursday morning and all the signs show that the mother and cub continue to be healthy, Senior Curator Brandie Smith said. The cub is also starting to develop dark markings in her fur around the eyes, ears and back.
"It's got a fat little belly," Smith said. "It's very active, very vocal."
The 10-day mark is critical for survival, Smith said. At that point, keepers started to gain more confidence about the cub's health.
"We're finally starting to really celebrate," she said.
China owns the pandas at the Smithsonian's National Zoo. The cub is expected to stay with her mother for a little over two years until she is weaned and stay at the zoo for about four years. Mei Xiang's only other surviving cub, a male named Tai Shan, was born in 2005 and was returned to China in 2010.
Mei Xiang gave birth to a cub last year after several years of failed breeding, but the cub died after six days. Its lungs hadn't fully developed and likely weren't sending enough oxygen to its liver.