A giant panda at the Atlanta zoo in Georgia, USA gave birth to twins on Monday, an apparent surprise to zoo officials who had been excitedly anticipating the birth of a single cub.

Twin cubs have never been born at Zoo Atlanta, zoo officials said. Lun Lun has given birth to three cubs there. All five offspring are the product of artificial insemination, according to zoo officials.

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. 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.