 This study will attempt to determine whether having gastric bypass surgery changes the way that the gut absorbs hormones from birth control pills. It is believed that, because gastric bypass surgery causes malabsorption of nutrients and some medications, the levels of birth control hormones after the surgery will be lower than in women before they have the surgery. The investigators will ask several women to take a pack of birth control pills before having bypass surgery, and then another pack several months after surgery. The investigators will measure hormone levels in the blood. The investigators will also measure outcomes that may tell us how well the birth control pills are working, such as ultrasounds of the uterus and ovaries, and examination of the cervix. 