IUDs are among the most effective forms of birth control available, with failure rates between 0.1% and 0.8% depending on the type. But several factors can compromise that protection, from the device physically shifting out of place to anatomical variations you may not know about. Here’s what can actually reduce your IUD’s effectiveness.
How Effective IUDs Are at Baseline
Hormonal IUDs (brands like Mirena, Liletta, Kyleena, and Skyla) have typical-use failure rates of 0.1% to 0.4%. The copper IUD (Paragard) has a slightly higher failure rate of 0.8%. These numbers are already very low, which means it takes something going genuinely wrong for an IUD to fail. But “very low” isn’t zero, and the causes of that gap are worth understanding.
Expulsion: When the IUD Comes Out on Its Own
The most common reason an IUD stops working is that it partially or fully slips out of the uterus. This is called expulsion, and it most often happens within the first three months after insertion. Over three years, roughly 10 out of every 100 IUD users will experience some degree of expulsion.
Your risk isn’t evenly distributed. Teenagers and young adults aged 14 to 19 have about double the expulsion rate of older users (18.8 per 100 versus 9.3 per 100). Having had a previous pregnancy also increases the risk: parous women had a cumulative expulsion rate of 11.4 per 100 compared to 8.4 per 100 for those who had never been pregnant. IUDs inserted immediately after an abortion also carry higher expulsion rates.
Body weight plays a role too. In a study of adolescents and young adults, those who experienced expulsion had an average BMI of 30.8 compared to 26.6 in those whose IUDs stayed in place. Moving from a healthy weight category to class 1 obesity was associated with roughly five times the odds of expulsion. Severe obesity carried about four times the odds.
An expulsion can be obvious (you feel or see the device) or subtle. If the IUD partially shifts, you might not realize it’s moved until your next check-up or until you notice changes in your bleeding pattern, new pelvic pain, or strings that feel longer or shorter than usual.
Malposition: Still Inside, but in the Wrong Spot
An IUD doesn’t have to fall out entirely to lose its effectiveness. If it shifts to the lower part of the uterus or into the cervix, it’s considered malpositioned, and about 73% of malpositioned devices end up in one of those two spots. This matters: women with malpositioned IUDs had a 19.2% pregnancy rate within two years compared to 10.5% for women whose devices were correctly placed.
You can’t feel malposition directly, but common signs include unexpected bleeding, cramping that doesn’t improve after the initial settling-in period, and strings that have changed in length or disappeared entirely. A follow-up visit is typically scheduled 4 to 6 weeks after insertion specifically to confirm the device is still in the right position.
Uterine Shape and Fibroids
Certain structural features of the uterus make it harder for an IUD to stay in the correct position. A retroflexed (backward-tilting) uterus was found in 7.6% of women with malpositioned IUDs, compared to just 1.8% in those with properly placed devices. Uterine anomalies overall, including septate uteri, bicornuate uteri, and fibroids, were present in nearly a third of malposition cases.
Fibroids are particularly relevant. It’s not the size of individual fibroids that matters most, but the total number and their location. Submucosal fibroids, which grow into the uterine cavity itself, showed a strong association with IUD displacement. Women without any uterine anomalies had roughly 40% lower odds of their IUD shifting out of place.
Perforation During Insertion
In rare cases, the IUD can push through the uterine wall during insertion. This happens in about 1.3 out of every 1,000 insertions. If the device migrates partially or fully outside the uterus, it obviously can’t provide contraception.
Timing and breastfeeding significantly affect this risk. Women who were breastfeeding at the time of insertion had a six-fold increase in perforation risk regardless of how long ago they delivered. The rate was 5.6 per 1,000 insertions for breastfeeding women within 36 weeks of delivery, compared to 1.7 per 1,000 for non-breastfeeding women in the same window. Waiting longer after delivery brought the numbers down for both groups.
Using It Past Its Approved Lifespan
Each IUD has a specific window of approved effectiveness. Mirena and Liletta are approved for eight years. Kyleena lasts five years. Skyla lasts three years. Paragard, the copper IUD, works for 10 years. Using a hormonal IUD beyond its approved duration means the hormone levels may have dropped too low to reliably prevent pregnancy. Paragard’s copper doesn’t deplete the same way, but the device itself can degrade over time.
The Waiting Period After Insertion
Copper IUDs are effective immediately after placement. Hormonal IUDs are not. Mirena and Kyleena can take up to seven days to reach full effectiveness. If you have unprotected sex during that window without backup contraception, pregnancy is possible. This is one of the most straightforward, and most avoidable, reasons an IUD can “fail.”
Do Medications Interfere?
This is where IUDs actually have a significant advantage over other hormonal methods. Enzyme-inducing medications, particularly certain anti-seizure drugs like phenytoin, carbamazepine, and topiramate, are well-documented to reduce the effectiveness of birth control pills. These drugs speed up hormone metabolism in the liver, clearing contraceptive hormones from the body before they can do their job.
Hormonal IUDs largely sidestep this problem. Because they release hormones directly into the uterus rather than relying on systemic blood levels, the American College of Obstetricians and Gynecologists specifically recommends the hormonal IUD as a contraceptive option for women taking enzyme-inducing medications. Copper IUDs, which contain no hormones at all, are completely unaffected by any medication interactions.
Signs Your IUD May Not Be Working
Most IUD failures come down to the device not being where it should be. The warning signs overlap considerably across the different causes:
- String changes: strings feel noticeably longer, shorter, or have disappeared entirely
- New or worsening pain: cramping that starts weeks or months after insertion, especially if the initial discomfort had already resolved
- Unusual bleeding: unexpected spotting or heavy bleeding outside your normal pattern
- Feeling the device: being able to feel hard plastic at your cervix, which can indicate partial expulsion
Checking your strings monthly, especially during the first few months, is the simplest way to catch a problem early. You can do this by reaching toward your cervix with clean fingers and feeling for the thin threads. If anything feels different from your baseline, an ultrasound can confirm whether the device is still properly positioned.