Combined oral contraceptives (COCs) remain a popular method of pregnancy prevention worldwide, but efficacy and failure rates can be difficult to determine, as real-word use does not always mirror clinical trials, wrote Mitchell D. Creinin, MD, of the University of California, Davis, and colleagues. Clinical trials include perfect use or method-failure rates, but data on pregnancy risk based on reported adherence alone are lacking, they said.
To assess the effects of missed pills on COC efficacy, the researchers reviewed data from a pair of parallel phase 3 trials, focusing only on adherence to the pill dosing regimen. The findings were published in Obstetrics & Gynecology.
The study population included 1,864 individuals from the United States and Canada, and 1,553 from Europe and Russia.
The participants were healthy, sexually active adults aged 16-50 years in monogamous relationships from 2016 through 2018 who agreed to used estetrol 15 mg and drospirenone 3 mg for up to 13 28-day cycles as their only contraceptive method. Condom use was permitted for protection against sexually transmitted infections if needed. The 28-day COCs included 24 hormonal tablets and 4 placebo tablets. Participants received written instructions for what to do it they missed pills.
The primary outcome was the relationship between missed pills and pregnancies.
A total of 31 pregnancies occurred across both studies; none of these occurred during cycles in which other contraception was used. Of 22 pregnancies in participants who reported taking all pills, 21 reported daily pill use during the cycle in which pregnancy occurred. One participant reported not taking one pill and one participant reported not taking two pills; neither correctly followed the instructions for missed pills.
Pregnancies occurred in .09% of cycles in which participants reported taking all pills, and in 0.25%, 0.83%, and 1.6% of cycles in which participants reported missing one pill, two pills, or more than two pills, respectively.
“Pregnancy rates exceeded 1% only in participants who did not correctly follow missed-pill instructions,” the researchers noted.
Pregnancy rates per cycle ranged from 0% to 0.21%, and 48.4% of the pregnancies occurred during the first four cycles of COC use. Approximately one-third (32.3%) of pregnancies occurred within the first week of a new pill pack.
“Fertilization does not appear to be related to the timing of missed pills within the cycle because pregnancy did not occur more frequently earlier in the cycle (after the placebo pills),” the researchers wrote in their discussion. This finding contradicts previous research suggesting that contraceptive failure rates decrease over the first year of use, they said. In addition, the formulation of the pill used may affect pregnancy rates when pills are missed, as some hormones have longer half-lives, they noted.
The study findings were limited by several factors, including the lack of adjustment for outcomes based on reported sexual activity per cycle, and by the reliance on self-reports.
However, the results were strengthened by the use of the clinical outcomes of pregnancy as the primary outcome, rather than characteristics and predictors of participants who missed pills, the researchers said.
The cycle-based methodology used in the current study may provide insight on the relationship between COC adherence and pregnancy risk that can inform future studies, they concluded.