Polycystic ovary syndrome (PCOS) causes harmless, small cysts in the ovaries and other symptoms. PCOS is caused by certain hormones being out of balance. The word "syndrome" means a group of symptoms. Women with PCOS may have no periods, irregular periods, or very long periods.
Women store their eggs in their ovaries. Each egg is in a capsule called a follicle. Normally during the reproductive years, one follicle grows to produce a mature egg each month. This egg is released during ovulation and the follicle dissolves.
With polycystic ovary syndrome (PCOS), the hormones that control ovulation are out of balance. These include estrogen, progesterone, and androgen. As a result, ovulation may not occur. Instead, the follicle stays enlarged. This is a fluid-filled sac called a cyst. Over time, the ovaries fill with many small cysts. This is why they are called "poly" or many "cystic" ovaries. In some women, the ovaries also make too much androgen.