The purpose of this study is to compare a low dose oral contraceptive (OC) given continuously (every day for three months) with the same low dose oral contraceptive given in an interrupted regimen (one week of inactive placebo pills each month) and with continuous placebo (inactive placebo given every day for three months). The primary hypothesis is that continuous OC will be significantly more effective in reducing premenstrual symptoms compared with either the interrupted OC or continuous placebo.