A newborn panda at the National Zoo in the US capital is healthy and \"very active, very vibrant and very pink,\" officials said Sunday. The good news comes a day after the cub\'s twin was stillborn, casting a shadow over the rare success. Keepers were able to briefly examine the surviving sibling -- born Friday -- just before 9 a.m. (1300 GMT), then return it to its mother Mei Xiang, National Zoo spokeswoman Jen Zoon told AFP. The sex and paternity of the tiny offspring, conceived via artificial insemination, will remain a mystery for two to three weeks until results of genetic testing come in.  The cub, which weighed in at a normal 4.8 ounces (137 grams), is approximately the size of a stick of butter. \"The cub is nursing and digesting well. It is very active, very vibrant and very pink,\" the National Zoo tweeted. The cub also has a steady heart beat, good lungs and is nursing well and passing fecals, according to Zoon. Until Sunday morning, workers weren\'t able to distract Mei Xiang and take the cub from her to do a physical exam. The zoo will follow Chinese tradition and leave the cub unnamed for approximately 100 days, until the beginning of December, Zoon said. After two years, the newest member of Washington\'s panda family will be sent to China and become part of a breeding program there, as part of a cooperative breeding agreement. Saturday\'s stillbirth was the fourth for Mei Xiang, who along with her male counterpart Tian Tian is on loan to the US zoo from China. The stillborn cub \"had developmental abnormalities, wasn\'t fully formed and was never alive,\" the zoo said in a tweet late Saturday. The surviving cub, however, is \"very robust and fully formed,\" Zoon said. Mei Xiang and Tian Tian attempted to conceive a cub naturally during their once-a-year mating ritual this winter. But after reproductive specialists determined that bid was unsuccessful, they performed artificial insemination using sperm samples from both Tian Tian and another male panda, Gao Gao, who resides at the San Diego Zoo. In September, Mei Xiang gave birth to a female cub, but it died six days later from liver damage due to underdeveloped lungs. Mei Xiang had five false pregnancies from 2007 to 2012. Her first cub, Tai Shan, was born in July 2005 as a result of artificial insemination. He now lives at the Bifengxia Panda Base in Ya\'an, China.