An ovarian follicle is a small, fluid-filled structure within the ovary that contains an immature egg, or oocyte. On ultrasound, follicles appear as small, round, anechoic cysts within the ovarian tissue. During a normal menstrual cycle, several follicles begin to develop each month, with one dominant follicle typically growing to about 18 to 25 millimeters before releasing its egg during ovulation. Counting and measuring follicles with ultrasound is important in fertility medicine for monitoring ovulation induction cycles, timing intrauterine insemination, and managing in vitro fertilization. The antral follicle count, which tallies the number of small follicles visible on ultrasound, is also used to assess a woman’s ovarian reserve.
Sonography Term