Polycystic ovary syndrome (PCOS) is a hormonal disorder that can affect a woman’s menstrual cycle, fertility, hormones, and appearance. PCOS is the most common cause of female infertility due to absent or irregular ovulation. PCOS may also cause small cysts in a woman’s ovaries. The exact cause of polycystic ovary syndrome isn’t known. However, some health conditions such as obesity and insulin resistance seem to contribute to its development.