Definition

A small, fluid-filled sac in the ovary that contains one immature egg. There are thousands of follicles in the ovaries. When an egg matures during a woman’s menstrual cycle, the follicle breaks open and releases the egg from the ovary for possible fertilization (the process by which an egg combines with sperm to form an embryo). Ovarian follicles may sometimes be checked to help diagnose and treat infertility.