An ovarian follicle is a small fluid-filled sac in the ovary, each holding one immature egg.
Why it matters for PCOS
In a normal menstrual cycle, several follicles begin to grow each month and usually one matures and releases its egg at ovulation. In PCOS this process is often disrupted, so many small follicles stay behind without maturing fully. On ultrasound this can appear as many small follicles around the edge of the ovary, known as polycystic ovarian morphology.
Polycystic ovarian morphology is only one of the Rotterdam criteria. A doctor reads follicle counts alongside symptoms, cycle pattern, and hormone tests, not as standalone proof of PCOS.
This glossary is education, not diagnosis. For your own situation, ask a doctor or pharmacist.