It took a month to figure out but it's now official. The twin baby pandas born at the Vienna zoo are brother and sister.

Schoenbrunn Zoo director Dagmar Schratter says photos of the twins have shown for the first time "that they are a boy and a girl." She said Tuesday that the fact that one of them is a female is particularly good news after all three cubs born previously were males.

The fact that it took a month to establish the cubs' genders is not surprising. Their birth was observed by camera only. And although they have grown substantially since then, the newborns were tiny, weighing only 100 grams (3.5 ounces) and with a length of around 10 centimeters (4 inches).