Giant panda gives birth to twins at Atlanta zoo Mon Jul 15, 2013 9:01pm EDT (Reuters) - A giant panda at the Atlanta zoo delivered an extra bundle of joy on Monday when she gave birth to twins, an apparent surprise to zoo officials who had been excitedly anticipating the birth of a single cub. Lun Lun delivered the tiny, hairless duo at 6:21 p.m. and 6:23 p.m. EDT, the first giant panda cubs to be born in the United States in 2013, Zoo Atlanta officials said. "We had no reason to expect twins, although we had not ruled them out," said Dwight Lawson, deputy director of Zoo Atlanta, adding that pandas regularly give birth to twins. On the few ultrasound images that Mom actually cooperated with, we only saw one developing fetus. Twin cubs have never been born at Zoo Atlanta, zoo officials said. Lun Lun has given birth to three cubs there. Giant pandas are an endangered species that make their homes in a few mountain ranges in central China. There are about 1,600 living the wild, and about 300 in captivity, mostly in China, zoo officials said. The sex of the cubs has not been determined yet, Lawson said. Panda cubs are about the size and weight of a stick of butter, and are pink, hairless and blind. Lun Lun weighs more than 200 pounds (91 kg), he said. Panda babies are about 1/900th the size of their mothers, according to the Smithsonian National Zoological Park. While pandas often give birth to twins in the wild, they are only able to raise one at a time - leaving the second twin to perish, Lawson said. But in a zoo setting, staff are able to incubate one cub while the mother takes care of the other, and then swap them out so she can have a chance at successfully raising both, Lawson said. "It's physically difficult for the mother panda to manipulate more than one cub," Lawson said. The zoo will work with experts in China to decide on how to name the twins once they turn 100 days old, Lawson said. For example, Taiwanese rockstar Su Huilun named Lun Lun, while U.S. actor Jack Black was at the naming of her most recent cub, Po, whose namesake character Black voiced in the "Kung Fu Panda" movies. The cubs are the fourth and fifth babies to born to Lun Lun and male Yang Yang, both 15. Their firstborn, Mei Lan, 6, is at China's Chengdu Research Base of Giant Panda Breeding, and the other two - 4-year-old Xi Lan and 2-year-old Po - are on site at the zoo but being prepared for a move to China later in the year. All five offspring are the product of artificial insemination, according to zoo officials. Reporting by Karen Brooks in Austin, Texas; Editing by Tim Gaynor and Sandra Maler