Objective: To assess ovarian suppression during oral contraceptive pill (OCP) use among normal-weight and obese women using two OCP doses.

Methods: This was a prospective, double-blind, randomized trial of two 21-day monophasic OCP formulations (20-microgram ethinyl estradiol [E2],/100-microgram levonorgestrel compared with 30-microgram ethinyl E2/150-microgram levonorgestrel) among normal-weight (body mass index 19.0-24.9) and obese (body mass index 30.0-39.9) women with regular menses and normal ovarian ultrasonography. Participants underwent transvaginal ultrasonography and phlebotomy twice weekly for 4 weeks during the third or fourth OCP cycle. We assessed OCP compliance using serum levonorgestrel levels. Outcomes included follicular development, endogenous E2 levels, ovulation, and self-reported bleeding patterns.

Results: Two hundred twenty-six women enrolled. One hundred eighty-one participants completed the study; we retained 150 consistent OCP users in the main analysis (96 normal weight, 54 obese). Consistent users of either OCP dose had substantial suppression of follicular development; obesity and follicular development were not related. Among the consistent OCP users, 2.7% ovulated during the study cycle (3 of 96 normal-weight and 1 of 54 obese participants). Two ovulations occurred with each OCP formulation. Inconsistent OCP use or nonuse during the study cycle was associated with more ovulation (P<.001). Normal-weight and obese participants had similar follicular development, endogenous estradiol levels, Hoogland scores, and bleeding patterns.

Conclusion: Normal-weight and obese participants who were consistent OCP users experienced substantial and comparable ovarian suppression during OCP use. Higher OCP failure rates among obese women reported elsewhere are thus unlikely to be attributable to physiological differences in OCP effect.

Clinical trial registration: ClinicalTrials.gov, www.clinicaltrials.gov; NCT00827632.

Level of evidence: : I.