In accordance with Chinese tradition, the cubs will only be named after 100 days because up to 50 percent of newborns do not survive, Dungl explained.
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But so far the siblings, which are being monitored around the clock, are doing very well, she added.
Female pandas are only fertile for a couple of days every year.
Yang Yang and her partner Long Hui, both aged 16, are already the proud parents of Fu Long, Fu Hu and Fu Bao, born in 2007, 2010 and 2013 respectively-and all conceived naturally.
The twins are expected to have their first public outing in four months' time.
Panda fans can follow their progress via videos and updates on the zoo's website: www.zoovienna.at.
VIEW PHOTOS: Panda Cub Bei Bei Grows Up