An ovarian follicle is a small, fluid-filled sac in the ovary that contains an immature egg. Each cycle, follicles grow, and usually one matures and releases its egg at ovulation.
Each ovary holds many follicles from birth. A follicle protects and nourishes its egg, and its cells make hormones, mainly estrogen. As a follicle grows, it produces more estrogen, which helps thicken the lining of the womb.
During the follicular phase, follicle-stimulating hormone (FSH) prompts a group of follicles to grow. Usually one becomes dominant while the rest stop developing. A surge of luteinizing hormone (LH) then makes the dominant follicle release its egg.
After ovulation, the empty follicle turns into the corpus luteum. This temporary structure makes progesterone to support a possible pregnancy.
Follicle numbers drop naturally with age. In fertility checks, a doctor may count small follicles on an ultrasound scan to estimate ovarian reserve. During fertility treatment, scans track how follicles respond to medicine. Many small follicles can also be one of the features doctors look at when assessing PCOS. Follicle counts alone do not predict whether someone will get pregnant, so results are best discussed with a fertility specialist.
Sources: MedlinePlus, Cleveland Clinic.
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