WATCH: Adorable Giant Panda Holds Rare Newly Born Twins in Vienna Zoo

It's a pleasant surprise for this giant panda: twins! Naturally conceived twins is a rare occurrence for this endangered species, but this one named Yang Yang gave birth to a pair in Vienna's Schoenbrunn Zoo.

According to a report from Reuters, Yang Yang actually gave birth over a week ago. However, the zookeepers all thought it was just one baby until CCTV monitoring over her enclosure revealed that she was cradling not one but two little cubs. A video from Daily Mail showed her cuddling with the two adorable pandas.

"It sounded as though two cubs were squealing, but we only ever saw one," Dagmar Schratter, zoo director, said in a statement to Reuters. "On Friday, the zookeepers were first able to make out two of them on the screen."

Yang Yang is currently monitored through CCTV in a private pen, so she doesn't get bothered or disturbed. She has three other offspring, all now in China.

It's not at all unusual for zookeepers not to get a glimpse of the baby (or babies) panda when one has just given birth. Cubs are constantly obscured from view as the mothers always keep them between their paws to warm them.

Newborn giant pandas are quite helpless. According to the National Zoo, giant panda cubs are only about 1/900th the size of its mother, which makes it the smallest mammal newborn relative to the mother's size, except for marsupials. They often only weigh roughly three to five ounces and are pink, hairless and blind -- somewhat unrecognizable from panda's lumbering black and white figure as an adult.

In the wild, cubs may stay with their mothers up to three years. The giant panda's slow breeding nature makes it difficult for the species to cope with the human-made threats to their population: illegal hunting and loss of their habitat, among others.