Estimating emergency contraception efficacy with levonorgestrel and copper intrauterine devices.

Objectives: This study aimed to determine observed vs expected pregnancy risk among levonorgestrel 52 mg and copper T380A intrauterine device (IUD) emergency contraception (EC) users.

Study design: This is a secondary analysis of participants seeking EC randomly assigned to a levonorgestrel 52 mg or copper T380A IUD. Participants had at least one episode of unprotected intercourse (UPI) in the 5 days preceding enrollment and reported all episodes in the preceding 5 days. We report the proportion of pregnancies prevented of those expected at 1 month following IUD insertion using an established approach that assigns pregnancy risk by menstrual cycle day of intercourse. We calculated proportion of pregnancies prevented using both the day of most recent UPI and all reported UPIs within 5 days of presentation.

Results: One pregnancy occurred among the 312 participants who received the levonorgestrel 52 mg IUD, with 137 (43.9%) reporting multiple episodes of UPI. No pregnancies occurred among the 318 participants who received the copper IUD, of whom 142 (44.6%) reported multiple episodes. In the levonorgestrel 52 mg IUD group, 14.8 pregnancies were expected using the most recent episode of UPI, and 22.0 pregnancies were expected using all episodes within 5 days of presentation. In the copper IUD group, we calculated 15.0 and 23.1 expected pregnancies, respectively. Levonorgestrel 52 mg IUDs prevented 93.2% to 95.7% of expected pregnancies, and copper IUDs prevented 100% of expected pregnancies.

Conclusions: The results of this analysis provide additional data quantifying the pregnancy risk reduction for both the levonorgestrel 52 mg IUD and copper IUD in a realistic population of EC users.

Implications: The levonorgestrel 52 mg IUD for emergency contraception prevented 93% to 96% of expected pregnancies using an established pregnancy risk method, while copper IUD users experienced 100% pregnancy prevention.