There are many potential mechanisms of action for the intrauterine device (IUD), which vary by type of IUD (inert, copper, or hormonal). This paper reviews the evidence for each potential mechanism of action. On the basis of available data for fertilization rates and clinical pregnancy rates, the relative contribution of mechanisms acting before or after fertilization were quantitatively estimated. These estimates indicate that, although prefertilization effects are more prominent for the copper IUD, both prefertilization and postfertilization mechanisms of action contribute significantly to the effectiveness of all types of intrauterine devices.
PMID 12501086 12501086 DOI 10.1067/mob.2002.128091 10.1067/mob.2002.128091 Stanford et al. 2002, Stanford 2002
Cite this article
Stanford, J. B., & Mikolajczyk, R. T. (2002). Mechanisms of action of intrauterine devices: update and estimation of postfertilization effects. American journal of obstetrics and gynecology, 187(6), 1699-1708. https://doi.org/10.1067/mob.2002.128091
Stanford JB, Mikolajczyk RT. Mechanisms of action of intrauterine devices: update and estimation of postfertilization effects. Am J Obstet Gynecol. 2002;187(6):1699-1708. doi:10.1067/mob.2002.128091
Stanford, J. B., and R. T. Mikolajczyk. "Mechanisms of action of intrauterine devices: update and estimation of postfertilization effects." American journal of obstetrics and gynecology, vol. 187, no. 6, 2002, pp. 1699-1708.
Backman T et al., 2004·American journal of obstetrics and gynecology
This study was undertaken to evaluate the pregnancy rate with the levonorgestrel intrauterine system (LNG IUS) and to analyze the outcome of pregnancies with the LNG IUS in situ in regular use. Questionnaires from 17,360 users of the LNG IUS were analyzed. With the consent of women reporting pregnancy during the use of the LNG IUS, related hospital records were reviewed concerning the pregnancies. Originally 132 pregnancies were reported. Medical records were reviewed from 108 of these women. In 64 pregnancies, conception occurred with the LNG IUS in situ. Thirty-three pregnancies were ectopic. The 5-year cumulative pregnancy rate per 100 users was 0.5 and the 5-year Pearl rate was 0.11. Pregnancy with the LNG IUS in situ is rare. Ectopic pregnancies constitute 53% of all pregnancies. Typical pregnancy symptoms occur during pregnancies with the LNG IUS. The importance to counsel about the risk of pregnancy before insertion is emphasized.
Michaelsen MP et al., 2025·BJOG: An International Journal of Obstetrics and Gynaecology
To study the association between previous use of levonorgestrel intrauterine system (LNG-IUS) and endometrial thickness (EMT) in women undergoing in vitro fertilisation (IVF)/intracytoplasmic sperm injection (ICSI) cycles. Multicentre historical cohort study. Eight Danish public and private fertility clinics. 12786 women aged 18-46 years contributing with an EMT measurement from 22 464 different IVF/ICSI treatment cycles between 2000 and 2021. Exposure was previous use of LNG-IUS, combined oral contraceptive pills (OCPs), progestogen-only pills (POPs), no/other contraception or combined, cumulated use of contraception when more contraceptives had been used during the inclusion period. LNG-IUS use was categorised into 0-3 years, >3-6 years, >6-9 years and >9 years. Mixed effect logistic regression adjusted for age, BMI, smoking, educational level, total FSH dose and fertility clinic was used. EMT (< 7 mm vs >= 7 mm). Statistically significantly higher odds of EMT >= 7 mm were found for OCPs [OR 3.53 (95% CI 1.29-9.65)], POPs [OR 6.43 (95% CI 1.45-28.63)] and no/other contraception [OR 6.67 (95% CI 2.37-18.74)] relative to LNG-IUS in IVF/ICSI cycles. All duration categories of ever use of LNG-IUS were associated with significantly lower odds of EMT >= 7 mm compared to no/other contraception. Previous use of LNG-IUS was associated with decreased endometrial growth in women undergoing IVF/ICSI.
To examine trends in national reporting of broken intrauterine devices (IUDs). We enumerated IUD device "breakage" reports in the Food and Drug Administration Adverse Event Reporting System from inception (1998) until February 2022. We explored associations of breakage with IUD type (copper versus hormonal), year reported, reporter (consumer versus clinician), and patient characteristics (age and weight). We identified 4144 breakage reports for copper versus 2140 for hormonal IUDs. Among the 170,215 adverse events reported, breaks were disproportionately reported for copper (9.6%) versus hormonal (1.7%) IUDs. National pharmacovigilance data suggests disproportionate breakage in copper versus hormonal IUDs though the true prevalence of breaks cannot be calculated from this dataset.
To assess the Pearl Index for risk of ectopic pregnancy in women using levonorgestrel-releasing intrauterine systems (LNG-IUS) with hormonal reservoirs of 13.5 mg, 19.5 mg, or 52 mg. This was a retrospective cohort study. Women diagnosed with an ectopic pregnancy in Stockholm County, Sweden, between January 1, 2014, and December 31, 2019, were identified through the electronic medical record system. The final analysis included 2,252 cases of ectopic pregnancy. Information on age, reproductive and medical history, as well as current use of contraception was retrieved. The time of intrauterine device (IUD) insertion before ectopic pregnancy and the numbers of sold LNG-IUS during the study period were used to calculate the incidence rate for ectopic pregnancy during use per 100 woman-years (Pearl Index). Among women with an ectopic pregnancy diagnosis, 105 presented with a known type of hormonal IUD in situ, of whom 94 were included in the calculations of the Pearl Index. The estimated Pearl Index for ectopic pregnancy was 0.136 (95% CI 0.106-0.176) for the LNG-IUS 13.5-mg, 0.037 (95% CI 0.021-0.067) for the LNG-IUS 19.5-mg, and 0.009 (95% CI 0.006-0.014) for the LNG-IUS 52-mg. With the 52-mg LNG-IUS as referent, the relative risk (RR) for ectopic pregnancy was higher during the first year for LNG 13.5-mg (RR 20.59, 95% CI 12.04-35.21), and for both 13.5-mg (RR 14.49, 95% CI 9.01-23.3) and 19.5-mg (RR 4.44, 95% CI 1.64-12.00) during the total study period. The absolute risk of ectopic pregnancy during the use of LNG-IUS at any doses was low. The results show that the lower the dose of the IUD, the higher the risk of an ectopic pregnancy. Higher-dose LNG-IUS should be considered when providing contraceptive counseling to a woman with known risk factors for ectopic pregnancy who are considering a hormonal IUD.