Nobody noticed that 24-year-old gorilla Shinda who is a bit overweight was pregnant. After several miscarriages, she was expected to remain childless.
Zoo director Miroslav Bobek says, "it seems that a miracle happens from time to time."
Bobek says the zookeepers "had given up any hope she could get a baby."
As the birth on Saturday afternoon was smooth and both mother and newborn have been doing well, visitors have been already allowed to see them. The baby's gender is not yet known.
The gorillas are among the most popular animals at the zoo. Tens of thousands of people watched the birth of another gorilla online in 2007.