Risk of unintended pregnancy based on intended compared to actual contraceptive use.

BACKGROUND: After initiating a new contraceptive method, the provider has little control of how or whether that method is used.

OBJECTIVE: We sought to compare unintended pregnancy rates by the initial chosen contraceptive method after counseling to traditional contraceptive effectiveness in the same study population.

STUDY DESIGN: The Contraceptive CHOICE Project provided reversible contraception to 9252 women at no cost during 2-3 years of follow-up. We performed 2 analyses of contraceptive efficacy in this prospective cohort: (1) intent-to-use (ITU), grouping participants based on their chosen method at enrollment; and (2) as-used, categorizing participant time according to the method used. In ITU analysis, switching of methods and method continuation were not considered, as we wanted to assess outcomes based on the method chosen at baseline. We used Cox proportional hazards models to compare rates of unintended pregnancy.

RESULTS: During 20,017 person-years, we identified 615 unintended pregnancies. In ITU analysis, pregnancy rates were 5.3, 5.5, 2.0, 1.7, and 1.9 per 100 person-years for women initiating oral, injectable, implantable, copper, and hormonal intrauterine contraception (IUC) at baseline, respectively. The adjusted hazard ratio for injectable contraception compared to hormonal IUC was 2.4 (95% confidence interval, 1.8-3.3). Delaying initiation of IUC or implantable contraception increased unintended pregnancies by 60% (adjusted hazard ratio, 1.6; 95% confidence interval, 1.2-2.0). In as-used analysis, pregnancy rates were 6.7, 1.6, 0.2, 0.6, and 0.2 per 100 person-years for women using oral, injectable, implantable, copper, and hormonal IUC, respectively.

CONCLUSION: Although highly effective in the as-used analysis, women initially choosing injectable contraception had pregnancy rates similar to oral contraception and significantly worse than IUC or implantable contraception. Despite switching and discontinuation, women choosing an IUC or implantable contraception at baseline were much less likely to have an unintended pregnancy compared to those selecting other methods.