Mirena (Levonorgestrel-releasing IUS) is a highly effective, long-acting reversible contraceptive (LARC) that provides pregnancy prevention for up to eight years. As an intrauterine system, it is placed directly into the uterus, offering continuous protection without requiring any daily user action. While this method is among the most reliable forms of birth control available, the risk of pregnancy, though very small, still exists. Understanding the specific failure rate and the factors that contribute to it provides a clearer picture of this device’s performance.
The Statistical Reality of Mirena’s Effectiveness
Mirena’s efficacy is measured using the Pearl Index, which calculates the number of pregnancies per 100 women over one year of use. For the Mirena IUS, the typical-use failure rate is exceptionally low. The failure rate is approximately 0.2% in the first year of use, with a cumulative failure rate of about 0.7% over five years of continuous use.
This high level of efficacy remains consistent even when the device is used for its full eight-year duration, where the cumulative failure rate is still less than 1%. Mirena’s performance in typical use is nearly identical to its performance in perfect use, which is a rare characteristic among contraceptives. For comparison, the typical-use failure rate for the birth control pill is about 7%, and for male condoms, it is around 13% over one year, largely due to user error. Mirena removes this risk of human error, making its failure rate significantly lower than methods that require daily action.
Understanding How Mirena Prevents Pregnancy
The mechanism by which the Mirena IUS prevents pregnancy is multi-layered, utilizing both hormonal and physical effects within the uterine environment. The device releases a continuous, low dose of the synthetic hormone levonorgestrel directly into the uterus. This localized delivery minimizes systemic side effects.
The primary hormonal action is the thickening of the cervical mucus, which creates a barrier that sperm cannot penetrate. The hormone also causes the lining of the uterus, the endometrium, to thin, making it less receptive to a fertilized egg. Mirena may also suppress the release of an egg, or ovulation, in some users, adding another layer of protection.
Scenarios That Increase Failure Risk
The majority of Mirena failures that result in pregnancy are due to its displacement or complete expulsion from the uterus, not device malfunction. Partial or complete expulsion occurs when the IUS moves out of its correct position within the uterine cavity, significantly reducing contraceptive protection. This risk is higher in the months following insertion, particularly if the insertion occurred immediately after childbirth.
Certain factors increase the likelihood of expulsion, including a history of heavy menstrual bleeding or a higher body mass index. Expulsion can be painless and go unnoticed, which is why users are advised to perform regular string checks. Signs that the device has moved include being unable to feel the threads, experiencing excessive pain, or having heavier or abnormal vaginal bleeding. A very rare risk is uterine perforation, which usually occurs during the insertion procedure and can also lead to a loss of efficacy.
Recognizing Pregnancy While Using Mirena
Recognizing a pregnancy while using Mirena can be uniquely challenging because the device frequently causes lighter periods or stops menstruation entirely in about 20% of users after one year. Therefore, the most common early sign of pregnancy—a missed period—may not be a reliable indicator for many Mirena users. Instead, users should be vigilant for other common pregnancy symptoms such as persistent nausea, unusual breast tenderness, or unexplained fatigue.
If pregnancy is suspected, taking a home pregnancy test and immediately contacting a healthcare provider is necessary. If a failure does occur, there is an increased risk that the resulting pregnancy will be ectopic. An ectopic pregnancy happens when the fertilized egg implants outside the uterus, typically in the fallopian tube, and this is a serious medical emergency. Symptoms suggesting an ectopic pregnancy include:
- One-sided pelvic pain.
- Abnormal vaginal bleeding.
- Dizziness.
- Intense abdominal pain.
These symptoms require immediate medical attention.