PCOS is the most common anovulatory disorder and the single most common hormonal cause of infertility. Anyone with ovaries can develop PCOS, and in fact an estimated 10% of women have been diagnosed with this disorder. In PCOS, a hormonal dysfunction results in the ovaries not being able to fully mature an egg. Follicles begin to develop and build up fluid, but do not progress to the point where they release an egg. As a result, ovulation is either absent or intermittent. Because of this, some of the semi-developed follicles turn into little fluid-filled sacs known as cysts. Without the progesterone that matured follicles would make, menstruation is irregular or perhaps stops altogether. On top of that, the cysts make androgens (masculinizing hormones, although they exist in all people in different proportions), which can prevent ovulation. It can be a vicious cycle.