Panda mama Mei Xiang gave birth to a rare healthy cub, then a surprise twin, on Saturday evening, leaving officials at the Smithsonian's National Zoo giddy—perhaps even moreso because the twins are off to a good start thanks to new techniques in panda care. Their sex won't be determined for another three to four weeks, according to the AP, and they will be named when they are 100 days old, per Chinese tradition. Zoo officials are working to make sure the cubs live at least that long. The team is using a swapping technique to care for the cubs, since Mei Xiang had trouble holding both of them at once. "It's very rare, obviously, for them to manage two cubs," says a panda biologist with the zoo. "If she were able to do it, we would certainly let her." Now, one cub nurses while the other is in an incubator.
The zoo's chief veterinarian admits that the next few weeks are critical. "A lot of things have to happen," he says. "Until the cubs are both out walking around, acting normal, being a panda, that's probably when we'll exhale." A serum was also developed from Mei Xiang's blood and fed to one of the cubs when it didn't get the chance to nurse. The birth of the cubs brings the number of pandas at the National Zoo to five. There's a male panda, Tian Tian, and Mei Xiang's 2-year-old daughter, Bao Bao.Mei Xiang was inseminated with semen from Tian Tian and from a panda in China; genetic testing will determine which of them is the father of her new twins. (Read more panda stories.)