Objective: To estimate real-life effectiveness of oral contraceptive pills by progestogen, length of pill-free interval, and body mass index while focusing on the effect of progestogens with a long half-life and on 24-day oral contraceptive pills regimens.
Methods: Outcome data from 52,218 U.S. participants in the International Active Surveillance of Women Taking Oral Contraceptives—a large, prospective, controlled, noninterventional, long-term cohort study with active surveillance of the study participants—were used to analyze contraceptive failure in association with oral contraceptive pills use. Low loss to follow-up is ensured by a comprehensive follow-up procedure. Contraceptive failure rates are described by Pearl Index and life-table analysis. Inferential statistics for contraceptive failure are based on Cox regression models.
Results: Analyses are based on 1,634 unintended pregnancies during 73,269 woman-years of oral contraceptive pills exposure. Life-table estimates of contraceptive failure for a 24-day regimen of drospirenone and ethinyl estradiol and 21-day regimens of other progestogens were 2.1% and 3.5% after the first study year, and 4.7% and 6.7% after the third year. The adjusted hazard ratio was 0.7 (95% confidence interval 0.6–0.8). Direct comparisons of the 24-day and 21-day regimens of drospirenone and norethisterone, respectively, showed also lower contraceptive failure rates for 24-day regimens. Contraceptive failure rates adjusted for age, parity and educational level showed a slight increase with higher body mass index.
Conclusion: The 24-day oral contraceptive regimens containing a progestogen with a long half-life show higher contraceptive effectiveness under routine medical conditions compared with conventional 21-day regimens. Obesity seems to be associated with a slight reduction of contraceptive effectiveness.
Clinical trial registration: ClinicalTrials.gov, www.clinicaltrials.gov, NCT00335257.
Level of evidence: II