No contraceptive method, including an IUD, prevents pregnancy 100 percent of the time. But IUDs come remarkably close. A large European surveillance study found a failure rate of roughly 0.06 per 100 woman-years for the hormonal (levonorgestrel) IUD and about 0.52 per 100 woman-years for the copper IUD, making both among the most reliable reversible contraceptives available.1PubMed. Comparative contraceptive effectiveness of levonorgestrel-releasing and copper intrauterine devices: the European Active Surveillance Study for Intrauterine Devices The gap between “nearly perfect” and “actually perfect” is small but real, and understanding what fills that gap matters if you’re making a decision about long-term birth control.
What the Failure Rates Actually Mean
When researchers talk about contraceptive failure rates, they typically use something called the Pearl Index, which counts unintended pregnancies per 100 woman-years of use. A Pearl Index of 0.06, the figure seen with hormonal IUDs, means roughly 6 pregnancies out of every 100,000 woman-years. For the copper IUD, the number is higher but still strikingly low. In practical terms, if 1,000 people used a hormonal IUD for a full year, you’d expect fewer than one pregnancy in the group. With the copper IUD, you might see a handful of pregnancies per thousand users over the same period.
These numbers hold up impressively well over time. A systematic review looking at extended use found that pregnancy rates during the first two years beyond the approved duration of the 52-mg levonorgestrel IUD and the T380A copper IUD remained low and comparable to rates during their labeled lifespan.2PubMed. Effectiveness and safety of extending intrauterine device duration: a systematic review Among nearly 500 levonorgestrel IUD users tracked into a sixth and seventh year, the failure rate stayed below half a pregnancy per 100 woman-years.3PubMed Central. Prolonged use of the etonogestrel implant and levonorgestrel intrauterine device: 2 years beyond Food and Drug Administration-approved duration So the device doesn’t suddenly stop working the day after its label expires, though the evidence for very long extensions is still limited.
How an IUD Prevents Pregnancy
The copper IUD and the hormonal IUD work differently, and those differences matter for understanding why failure happens. All IUDs trigger a mild inflammatory response inside the uterus that makes the environment hostile to sperm.4PubMed. Copper-T intrauterine device and levonorgestrel intrauterine system: biological bases of their mechanism of action With the copper IUD, copper ions released into the uterine and tubal fluid are directly toxic to sperm, dramatically reducing the chances of fertilization. The hormonal IUD adds a local dose of levonorgestrel that thickens cervical mucus, making it harder for sperm to enter the uterus in the first place, and thins the uterine lining. Both types may also interfere with implantation, though how much that contributes to their overall effect remains uncertain.5Human Reproduction Update. Intrauterine devices and intrauterine systems
Because these mechanisms are local, acting right where the device sits, they’re largely independent of what’s happening in the rest of your body. This gives IUDs a major advantage over methods that rely on circulating hormones and makes them resistant to the kinds of interference that trip up pills or patches.
Why the Rare Failures Happen
When an IUD does fail, the device has usually moved. The main culprits are expulsion, where the IUD partially or completely slides out of the uterus, and malposition, where it shifts enough to lose proper contact with the uterine lining. Less commonly, the device can perforate the uterine wall and end up outside the uterus entirely. All of these can reduce or eliminate the contraceptive effect.6PubMed. Uterine dimensions and intrauterine device malposition: can ultrasound predict displacement or expulsion before it happens? Perforation is rare, and when it does occur, loss of contraceptive protection is the most clinically significant consequence alongside the risk of organ injury.7PubMed Central. Pregnancy with Intrauterine Device Perforation: A Case Report
Uterine dimensions play a role. Research has found that certain cavity measurements are associated with a higher chance of expulsion or displacement. For one type of copper IUD, a particular transverse diameter above a threshold roughly doubled the odds of expulsion; for another type, the odds were about five times higher in women whose cavities exceeded the relevant size cutoff.8PubMed. Dimensions of the endometrial cavity and intrauterine device expulsion or removal for displacement: a nested case-control study This is essentially a fit problem: the device needs to sit snugly in the uterine cavity, and when the two don’t match well, it’s more likely to shift.
Who Is More Likely to Have an Expulsion
Age is one of the strongest predictors. Teenagers and younger users face a noticeably higher risk. In one large analysis, those aged 14 to 19 had roughly two to three times the hazard of expulsion compared with older users, depending on IUD type.9PubMed Central. Association of Age and Parity With Intrauterine Device Expulsion Interestingly, the same study found that nulliparous users of the levonorgestrel IUD actually had lower expulsion rates than those who had previously given birth, which runs counter to the lingering myth that IUDs are only appropriate for people who have already had children.
A history of previous IUD expulsion is another strong red flag. A large case-control study found that women who had expelled an IUD in the past had more than three times the odds of a future IUD failure.10Human Reproduction. Risk factors for IUD failure: results of a large multicentre case–control study The same study found that being older than 35 was associated with a lower risk of failure, and that gynecological conditions like fibroids, polyps, or a history of miscarriage did not increase the risk. On a related note, a separate nested case-control study found that uterine position and cavity depth measured at insertion were not associated with pregnancy risk in IUD users, suggesting that standard pre-insertion assessments should not be used to exclude otherwise eligible candidates.11PubMed. Uterine factors and risk of pregnancy in IUD users: a nested case-control study
How Insertion Timing Affects Success
If you’re getting an IUD placed after giving birth, when it goes in makes a big difference. A meta-analysis of postpartum IUD studies found that expulsion rates varied dramatically by timing: about 2 percent for placements done four or more weeks after delivery, around 10 percent for placements within ten minutes of the placenta coming out, and nearly 30 percent for placements in the window between those two time points.12PubMed Central. Intrauterine Device Expulsion After Postpartum Placement: A Systematic Review and Meta-analysis Vaginal delivery was also associated with higher expulsion risk than cesarean delivery when the IUD was placed in the early postpartum window.
A large cohort study confirmed this pattern and added some useful detail. The five-year expulsion rate was highest for insertions in the first zero to three days after delivery, at about 11 percent, and lowest for insertions at six to fourteen weeks postpartum, at roughly 3 percent. That study also found that breastfeeding was associated with about a 30 percent lower risk of expulsion compared with not breastfeeding.13JAMA Network Open. Association of the Timing of Postpartum Intrauterine Device Insertion and Breastfeeding With Risks of Intrauterine Device Expulsion The clinical trade-off here is real: placing an IUD right after delivery is convenient and ensures you leave the hospital with contraception in place, but it comes with a meaningfully higher chance the device won’t stay put. Waiting six or more weeks brings the expulsion risk close to that of a standard non-postpartum insertion.
Body Weight and Drug Interactions
One of the genuine practical advantages of IUDs over many other contraceptive methods is that body weight does not reduce their effectiveness. Because the device works locally inside the uterus rather than relying on drug levels in the bloodstream, body mass index doesn’t dilute the effect the way it can with oral contraceptives. An analysis from the Contraceptive CHOICE Project found that three-year failure rates for both copper and hormonal IUDs in overweight and obese women were below 1 per 100 woman-years, with no variation by BMI category.14PubMed Central. OBESITY AND CONTRACEPTION Insertion can be more technically challenging at higher body weights, but effectiveness after placement is the same.15PubMed Central. Contraception and the obese woman
Drug interactions are similarly a non-issue for most IUD users. Medications that induce liver enzymes, like certain anti-seizure drugs and antiretrovirals, can reduce the effectiveness of hormonal contraceptives that depend on systemic absorption. But the levonorgestrel IUD works primarily through its local effect on the endometrium, so hepatic metabolism is largely irrelevant. Research has found no drug interactions between either type of IUD and antiretroviral or anti-epileptic drugs.16SASGOG. Hormonal Contraception in Women Taking Medications with Potential Drug Interactions This makes IUDs a particularly strong option for people on medications that would interfere with pills, patches, or rings.
How IUDs Compare to Sterilization and Implants
The comparison people often wonder about is whether surgical sterilization, which feels permanent and therefore “guaranteed,” is actually more effective than an IUD. The answer may surprise you. A study comparing real-world outcomes found that within one year, pregnancy rates were similar after tubal ligation, levonorgestrel IUD placement, and copper IUD placement, hovering around 2 to 3 percent in unadjusted numbers. After adjusting for confounders, levonorgestrel IUD users were actually less likely to become pregnant than those who had undergone tubal ligation.17PubMed Central. Comparative Effectiveness and Safety of Intrauterine Contraception and Tubal Ligation The copper IUD performed similarly to sterilization.
Compared with the contraceptive implant, IUDs also perform on par. A study examining failure rates across BMI categories found no differences in contraceptive failure between implants and IUDs in normal-weight, overweight, or obese women.18PubMed Central. Contraceptive Failure Rates of Etonogestrel Subdermal Implants in Overweight and Obese Women If you’re choosing between these long-acting reversible options, effectiveness alone isn’t a strong reason to pick one over the other. Other factors, like whether you want a hormonal or non-hormonal method, how you feel about potential side effects, and how long you want the device to last, are more useful for making the decision.
Checking That Your IUD Is in Place
Because the main failure mode is the device moving out of position, knowing how to check whether yours has shifted matters. After insertion, most providers teach you to feel for the IUD strings, thin threads that extend through the cervix into the upper vagina. If you can’t feel them, the device may have expelled, or the strings may simply have curled up inside the cervical canal. A study of women with non-visible IUD strings found that ultrasound confirmed the device was still properly in place about 98 percent of the time. In about 1 percent of cases, the device had expelled, and in less than 1 percent, it had perforated into the pelvis.19PubMed. Management of missing strings in users of intrauterine contraceptives So missing strings are worth checking out, but they’re usually not a sign of a problem.
Most providers recommend checking the strings once a month, after your period is a good time to remember. If you notice the strings are suddenly longer than before, or if you can feel hard plastic at the cervical opening, the IUD may be partially expelled and you should use backup contraception until you’re evaluated. Between string checks, unusual cramping, heavy bleeding that’s new for you, or pain during intercourse can all be signs that something has shifted.
What Happens If You Get Pregnant With an IUD
In the uncommon event that pregnancy does occur with an IUD in place, the situation requires prompt medical attention. One concern is ectopic pregnancy, where the fertilized egg implants outside the uterus, usually in a fallopian tube. A large recent study found that the absolute risk of ectopic pregnancy in IUD users is low across all types but is not zero. At one year, ectopic pregnancy rates ranged from about 0.04 per 100 person-years with the highest-dose hormonal IUD to roughly 0.18 per 100 person-years with the lowest-dose version. Copper IUD users fell in between, at about 0.07 per 100 person-years.20PubMed. Intrauterine Devices and Risk of Ectopic Pregnancy Additional risk factors for ectopic pregnancy in IUD users include a history of tubal damage, prior spontaneous abortion, and pelvic pain after IUD insertion.21PubMed. Risk factors for extrauterine pregnancy in women using an intrauterine device
For intrauterine pregnancies with an IUD still present, the evidence strongly favors removing the device if at all possible. A systematic review found that women who kept the IUD in place had substantially higher risks of miscarriage, preterm delivery, and infection compared with those who had it removed.22PubMed. Pregnancy outcomes with an IUD in situ: a systematic review With the levonorgestrel IUD specifically, eight out of ten pregnancies with the device left in place ended in spontaneous abortion. Removal doesn’t eliminate the elevated risk entirely, but it improves outcomes considerably. One study found that the combined rate of adverse pregnancy outcomes dropped from about 63 percent in women who retained the IUD to about 37 percent in those who had it removed.23PubMed. Outcome of intrauterine pregnancies with intrauterine device in place and effects of device location on prognosis
When removal requires a more involved approach, hysteroscopic removal has shown a high success rate of over 96 percent. In cases where the procedure succeeded, roughly 82 percent of women went on to deliver at term.24PubMed Central. Hysteroscopic Removal of Intrauterine Device in Pregnancy: A Scoping Review to Guide Personalized Care The bottom line: a positive pregnancy test with an IUD warrants an urgent visit to your provider to confirm the location of the pregnancy and discuss removal.
Fertility After IUD Removal
A persistent worry among IUD users is that the device might impair fertility down the road. The evidence on this is reassuring. A pilot study tracking women after IUD removal found that 81 percent became pregnant within 12 months, compared with 70 percent of non-IUD users trying to conceive over the same period, and there was no statistical difference in time to pregnancy between the two groups.25PubMed Central. Fertility after intrauterine device removal: a pilot study A much larger study of over 1,700 women found that about 80 percent became pregnant after removal, with roughly 88 percent of those pregnancies occurring within the first year.26PubMed. Fertility in older women following removal of long-term intrauterine devices in the wake of a natural disaster That study involved older women who had used IUDs long-term, and while it did note a higher rate of fertility difficulties in this population, age itself is the obvious confounder. The IUD does not appear to add independent barriers to conception once it’s out.
This matters because the fear of future infertility is one of the most commonly cited reasons people hesitate to choose an IUD, especially younger users considering long-acting contraception for the first time. The inflammatory response that makes the uterus inhospitable to sperm while the device is in place resolves quickly after removal. Ovulation and normal cycling typically return within a month.
When Effectiveness Numbers Feel Misleadingly Low
If you’ve read this far and noticed that one real-world study reported pregnancy rates of around 2 to 3 percent within a year of IUD placement, you might wonder how that squares with the very low Pearl Index figures cited earlier. The discrepancy comes down to study design and population. Clinical trials and surveillance studies that produce Pearl Indices tend to enroll motivated participants, confirm proper placement, and track device continuity closely. Real-world data from insurance claims or electronic health records captures everyone, including people whose IUDs were silently expelled or who had complications that led to early removal. A one-year pregnancy rate of roughly 2 to 3 percent in an administrative dataset likely includes women who were no longer protected by an IUD at all, not just women in whom the method failed despite correct placement.
This is an important distinction. The IUD itself, sitting correctly in the uterus, performs extremely well. The real-world number includes everything that can go wrong between insertion and the one-year mark: partial expulsions, removals for side effects with a gap before the next method starts, and the fact that some pregnancies in these datasets may have been conceived after the IUD was already gone. When clinicians say an IUD is “over 99 percent effective,” they are talking about the device working as designed, not the entire messy experience of being a contraceptive user in the real world.
Using an IUD Beyond Its Labeled Duration
The approved lifespan for different IUDs ranges from three years for the lowest-dose hormonal versions up to ten or even twelve years for certain copper models. But approval labels are based on the length of the clinical trials that were conducted, not necessarily on the point where the device stops working. The 52-mg levonorgestrel IUD, for instance, was originally approved for five years and was later extended to eight in some countries as longer data became available. Research following users into a sixth and seventh year found failure rates of about 0.25 and 0.43 per 100 woman-years respectively, which is still extremely low and comparable to the rates seen during approved use.3PubMed Central. Prolonged use of the etonogestrel implant and levonorgestrel intrauterine device: 2 years beyond Food and Drug Administration-approved duration An earlier study from the same research group found a similar pattern, with one pregnancy among 263 levonorgestrel IUD users tracked beyond the labeled duration.27PubMed Central. Use of the Etonogestrel Implant and Levonorgestrel Intrauterine Device Beyond the U.S. Food and Drug Administration–Approved Duration
For the copper T380A, studies in various populations have shown continued efficacy well past the ten-year mark. A one-year cohort in Tanzania found a method failure rate of roughly 2 per 1,000 for postpartum copper IUD users, consistent with the very low expected rates for this device type.28PubMed Central. A one-year cohort study of complications, continuation, and failure rates of postpartum TCu380A in Tanzania If you’re approaching the end of your IUD’s labeled lifespan and can’t get an appointment right away, the evidence suggests the device is unlikely to suddenly fail. That said, discussing extended use with your provider is still worthwhile, since individual factors like your IUD type, age, and plans for future pregnancy all play into the decision.