The ovaries that are used for reproduction in a woman develop a mature egg suitable for fertilization during menstruation. The egg develops and matures in a fluid-filled sac called a follicle. A woman's follicle is plucked every month, and it grows and ovulates. In women with PCOS, this follicle cannot be selected, so ovulation does not occur every month. On ultrasound, the ovary shows many pustules containing immature eggs, that is, many cysts.