RESUMO
INTRODUCTION: The objective was to investigate the Pearl index for ectopic pregnancy in women using the levonorgestrel intrauterine system (LNG-IUS) at the time of conception. MATERIAL AND METHODS: This was a substudy of a hospital-based historical cohort including women with an ectopic pregnancy diagnosed in 4 referral hospitals in Stockholm, Sweden between December 2013 and April 2017. Study participants and type of contraception at time of conception were identified through the electronic medical chart system. Cases were defined as women with LNG-IUS 13.5 or 52 mg in situ at the time of diagnosis of ectopic pregnancy who had their LNG-IUS inserted 1 January 2014 or later. Outcome was defined as the incidence rate for ectopic pregnancy during use per 100 woman-years (Pearl index) with 95% confidence interval (95% CI). RESULTS: The study includes 35 cases with LNG-IUS 13.5 mg and 13 cases with LNG-IUS 52 mg, which occurred in 18 488 (13.5 mg) and 50 246 (52 mg) exposed woman-years. Pearl index for ectopic pregnancy was estimated at 0.19 (95% CI 0.15-0.28) for LNG-IUS 13.5 mg and to 0.006 (95% CI 0.00-0.01) for LNG-IUS 52 mg. The most cases for the LNG-IUS 13.5 mg (n = 26; 74%) occurred during the first year of use with a corresponding Pearl index at 0.23 (95% CI 0.14-0.35). CONCLUSIONS: The absolute risk of ectopic pregnancy during the use of LNG-IUS remains low. Our method may underestimate as well as overestimate the Pearl index for ectopic pregnancy. The findings underline the importance of identifying risk factors for ectopic pregnancy at insertion, and to rule out ectopic pregnancy for all women presenting with abdominal discomfort during the use of LNG-IUS. Longer observation periods and prospective studies in a general population are a priority for further research.