There is a large gap between lowest expected failure rates of about 1.5% and typical use failure rates of about 7% per year. This gap may be due to incorrect use or to decreased OC effectiveness in population subgroups. Recent reports suggest greater OC failure among heavier women, particularly those using the lowest doses. The prevalence of obesity in the US population has recently increased to about 23% in women aged 2029, peak years for OC use. OC physiology and effectiveness have not been evaluated in obese women. We propose a double blind randomized clinical trial to include normal weight and obese women who have normal ovulatory function at baseline; we will randomize women to 2 widely used OCs and evaluate ovarian follicle development and circulating progesterone to assess ovarian suppression during OC use.