Polycystic Ovary Syndrome, or PCOS, is a condition caused by an imbalance of female sex hormones that results in a decreased ability for the ovaries to release mature eggs. PCOS affects between 5% and 10% of the population. Rather than ovulating like normal, the eggs of a woman with PCOS tend to stay in the ovary and form cysts. PCOS is more frequent in women who are overweight, have diabetes, or have a family history of PCOS. Read more.