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.