The ovaries are two small organs, one on each side of a woman's uterus. A woman's ovaries have follicles, which are tiny sacs filled with liquid that hold the eggs. These sacs are also called cysts. Each month about 20 eggs start to mature, but commonly only one becomes dominant. As the one egg grows, the follicle accumulates fluid in it. When that egg matures, the follicle breaks open to release the egg so it can travel through the fallopian tube for fertilization. When the single egg leaves the follicle, ovulation takes place.

In women with PCOS, the ovary doesn't make all of the hormones it needs for any of the eggs to fully mature. They may start to grow and accumulate fluid. But no one egg becomes large enough. Instead, some may remain as cysts. Since no egg matures or is released, ovulation does not occur and the hormone progesterone is not made. Without progesterone, a woman's menstrual cycle is irregular or absent. Also, the cysts produce male hormones, which continue to prevent ovulation.

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