You need an egg to conceive a baby. In the usual process, an egg is produced from a follicle in the ovary and released into the fallopian tube. But in a woman with polycystic ovarian syndrome, or PCOS, this process is disrupted by a hormone imbalance. The eggs in the follicles of the ovary never mature -- they simply become cysts, and no egg at all is released.
PCOS causes the ovaries produce more androgens than usual. Androgens, such as testosterone, are known as the "male hormones," but that's a misnomer. Men and women both make them.
This "too much testosterone" condition doesn't just affect egg production -- it can also cause unwanted hair growth (in the form of a mustache, for example), smaller breasts, acne and thinning hair. Obesity is also a hallmark of PCOS, which is why the first advice a doctor often gives to a woman with PCOS hoping to get pregnant is to lose weight and make other lifestyle changes. As far as drugs and procedures go, the best treatment researchers have found so far is clomiphene, a drug that stimulates ovulation.