Why do women with polycystic ovary syndrome (PCOS) have trouble with their menstrual cycle?

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 usually only one matures fully. As this one egg grows, the follicle accumulates fluid in it. When that egg matures, the follicle breaks open to release it (ovulation). The egg then travels through the fallopian tube, where fertilization can take place.

In women with PCOS, the ovary doesn't make all of the hormones it needs for any of the eggs to fully mature. Several follicles may start to grow and build up fluid, but no one follicle becomes large enough. Instead, some follicles may remain as very small cysts (thus the term polycystic ovary).   Thus, since no follicle becomes large enough and no egg matures or is released, ovulation does not occur, and so the hormone progesterone is not made. Without progesterone, a woman's menstrual cycle is irregular or absent. Also, the cysts make male hormones, which also prevent ovulation.

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Information is provided by:  The National Women's Health Information Center which is Sponsored by the Office on Women's Health in the U.S. Department of Health and Human Services