Cysts are fluid-filled sacs that can form in the ovaries. They are very common in women, especially during the childbearing years. Often, they form during the process of ovulation. It can happen when the egg is not released or when the sac (or follicle) holding the egg doesn't dissolve after the egg is released. Ovarian cysts usually cause no symptoms and dissolve on their own. They can, though, create a dull ache or a sharp pain if the cyst is large and it ruptures.