What Causes Cyst In Ovary?

hi,my wife's scan showed up a cyst in her left ovary about 5cm x 4cm.

we are very scared with this and are waiting to see the specialist who has given us an appoinment for next week.is the size of the cyst dangerous. She has never had any discomfort in her stomach.However her stomach has noticably bloated over the last 6 months and she has gained 3 inches.