A randomized trial of three oral contraceptives: Comparison of bleeding patterns by contraceptive types and steroid levels

OBJECTIVE: Our purpose was to determine the relationship between bioavailability of contraceptive steroids and bleeding patterns. STUDY DESIGN: A randomized clinical trial evaluated 192 women on 50 μg of ethinyl estradiol and 1.0 mg of norethindrone (OC1), 35 μg ethinyl estradiol and 1.0 mg of norethindrone (OC2), and 35 μg ethinyl estradiol and 0.5 mg norethindrone (OC3) over nine cycles. RESULTS: intermenstrual bleeding rates were higher for OC3 when compared with OC1 (p = 0.01). The number of intermenstrual bleeding days was highest for OC3 (p = 0.001) and higher for OC2 when compared with OC1 (p < 0.006). The onset of withdrawal bleeding occurred faster in OC3 patients (p < 0.02). Bioavailability of both contraceptive steroids as measured by baseline values and 1-hour slopes did not correlate with bleeding patterns at 3, 6, and 9 months of use. CONCLUSION: These data suggest that differences in biologic responses associated with pill use cannot be explained solely on the basis of these particular hormone measurements.