The rate of unintended pregnancy in the United States is 50%. About half of unintended pregnancies result from contraceptive failure, largely because of incorrect or inconsistent use of contraception. There is a high rate of failure with use of the oral contraceptive pill, the most common contraceptive method in the United States, because of lack of compliance to daily dosing. In contrast, the failure rates with long-acting reversible contraceptive methods, including intrauterine devices (IUDs) and subdermal implants (which are not user dependent) are less than 1%. Despite the proven effectiveness of IUDs in women and adolescents of all ages, they are used by only 5.5% of women who use contraception in the United States. Compared to the United States, rates of unintended pregnancy are lower in the United Kingdom and France where IUDs are used more. There are only limited prospective data on the effectiveness of contraceptive methods in large US populations.
The aim of this large prospective cohort study was to compare the failure rates of long-acting reversible contraception (IUDs and implants) with those of other commonly prescribed contraceptive methods (pills, transdermal patch, vaginal ring, and depot medroxyprogesterone acetate [DMPA] injection). All participants received contraceptive counseling and a contraceptive method of their choice at no cost. The primary study outcome was contraceptive failure; a secondary outcome was the rate of failure stratified by age (<21 vs ≥21 years). Participants were followed up prospectively using telephone interviews for up to 3 years. The final multivariate model included adjustment for potential covariables.
A total of 334 unintended pregnancies were identified among the 7486 participants included in the analysis. The contraceptive failure rate was substantially higher among participants using pills, patch, or ring (4.55 per 100 participant-years) compared with those using long-acting reversible contraception (0.27 per 100 participant-years); the hazard ratio (HR) after adjustment for age, educational level, and history of unintended pregnancy was 21.8, with a 95% confidence interval of 13.7 to 34.9 (P < 0.001). The risk of unintended pregnancy was nearly 2-fold higher among subjects less than 21 years of age who used pills, patch, or ring compared with older women using this same method (HR adjusted for educational level and history of unintended pregnancy, 1.9; 95% confidence interval, 1.2-2.8; P = 0.02). The rate of contraceptive failure was similarly low among participants using an IUD or implant and those using DMPA injection, regardless of age.
These findings demonstrate that long-acting reversible methods of contraception are more effective in preventing unintended pregnancy than contraceptive pills, patch, or ring in both young and older women.