IUDs are among the most effective forms of birth control available, but a few specific factors can reduce how well they work. The main risks involve the device moving out of position, certain medications interfering with hormonal IUDs, and uterine anatomy that makes proper placement difficult. Here’s what actually matters and what doesn’t.
How Effective IUDs Are at Baseline
To understand what “less effective” means, it helps to know the starting point. Hormonal IUDs have a pregnancy rate of about 0.3%, while copper IUDs sit around 1.2% to 2.0% depending on the size of the device. Both types are over 98% effective, placing them in the top tier of contraceptive methods alongside implants and sterilization. Anything that pushes those numbers higher is worth knowing about.
The Biggest Risk: Your IUD Moves
The single most important factor in IUD effectiveness is whether the device stays where it was placed. An IUD works by sitting at the top of the uterus, near the fundus. If it shifts downward into the cervical canal or partially comes out, it can no longer do its job properly. Copper IUDs that sit more than 3 millimeters from the fundus are generally considered too low to guarantee protection.
A study published in Obstetrics & Gynecology found that women with a malpositioned IUD were twice as likely to become pregnant within 12 months compared to those with a correctly positioned device (10% vs. 5%). Interestingly, the increased pregnancies weren’t happening while the IUD was still in place. Instead, malpositioned IUDs were more likely to be removed or expelled, leaving women unprotected. So the real danger isn’t a slightly shifted IUD quietly failing. It’s that a poorly positioned IUD is more likely to come out entirely.
Complete expulsion, where the IUD falls out of the uterus, is the most obvious way it stops working. Overall expulsion rates vary, but one large study of IUDs placed right after delivery found a rate of 14.7%. The risk was much higher after vaginal delivery (22.8%) compared to cesarean delivery (5.2%). Women under 20 had roughly five times the odds of expulsion compared to older women. If you had your IUD placed shortly after giving birth, checking that it’s still in position is especially important.
Signs Your IUD May Have Shifted
About half of women with a displaced IUD have no symptoms at all, which is why routine string checks matter. The other half typically notice unusual bleeding, cramping, or pain. Bleeding disturbances are the strongest predictor of malposition. Pain that lasts more than three days after insertion, or that continues for weeks, can also signal a problem. Some women only notice pain during sex.
If you can feel the hard plastic of the IUD at the opening of your cervix, or if your strings suddenly feel much longer or shorter than usual, the device has likely moved. An ultrasound is the definitive way to confirm position.
Medications That Interfere With Hormonal IUDs
Certain drugs speed up how quickly your liver breaks down hormones, which can reduce the effectiveness of hormonal contraceptives. These include some epilepsy medications (like carbamazepine and phenytoin), the antibiotic rifampin used for tuberculosis, and certain antiretroviral drugs. For oral contraceptives, these medications can reduce hormone levels by 40% to 80%, which is dramatic enough to cause failures.
Here’s where IUDs have a significant advantage, though. Because hormonal IUDs release their hormone directly into the uterus rather than relying on levels in the bloodstream, they appear to be largely protected from these drug interactions. A recent review comparing contraceptive failure rates across different methods found that liver enzyme-inducing drugs increased pregnancy risk for pills and implants but not for intrauterine or vaginal devices. So while this concern applies strongly to the pill and somewhat to the implant, the hormonal IUD is considered a safer choice for women on these medications.
Copper IUDs work through a completely non-hormonal mechanism, so no medication interaction affects their contraceptive function.
What About NSAIDs Like Ibuprofen?
There’s a persistent belief that anti-inflammatory painkillers reduce copper IUD effectiveness by suppressing the inflammatory response the device relies on. A Cochrane review looked at this directly and found that NSAIDs like ibuprofen, naproxen, and mefenamic acid are effective at reducing both bleeding and pain associated with IUD use. The review actually recommends NSAIDs as first-line treatment for IUD-related side effects. There is no evidence that taking ibuprofen for cramps after insertion compromises your IUD’s ability to prevent pregnancy.
Uterine Shape and Fibroids
An IUD needs a normally shaped uterine cavity to sit in the right position. Fibroids, which are noncancerous growths in the uterine wall, can distort the cavity and make it harder for the device to stay put. Studies show expulsion rates of 0% to 20% among women with fibroids, with some evidence that rates are higher than the 0% to 3% range seen in women without them. The location and size of the fibroids matter: a small fibroid on the outer wall of the uterus may cause no issues, while one bulging into the cavity can push the IUD out of position or prevent proper placement altogether.
Women with a uterine cavity that is unusually small or has a structural variation may also have trouble retaining an IUD. If the device is too large for the cavity, it can embed in the wall, causing pain and bleeding, or even perforate the uterus in rare cases.
Body Weight Does Not Reduce IUD Effectiveness
Unlike oral contraceptives and emergency contraception pills, IUDs work just as well regardless of your weight. A systematic review found that neither hormonal nor copper IUD pregnancy rates were affected by BMI. This is because IUDs act locally inside the uterus rather than relying on hormone levels in the bloodstream, which can be diluted in larger bodies. One study did note slightly higher expulsion rates in women with class III obesity (BMI of 40 or above), but even in that group, there was no increase in actual pregnancy rates. For comparison, the emergency contraception pill levonorgestrel is over four times more likely to fail in obese women, while the copper IUD maintains its roughly 0.14% failure rate regardless of weight.
Timing of Insertion Matters for Hormonal IUDs
Copper IUDs start working immediately. The copper creates an environment in the uterus that is toxic to sperm from the moment of placement, so no backup contraception is needed.
Hormonal IUDs are different. If yours is placed within the first seven days of your period, it’s effective right away. If it’s placed at any other point in your cycle, you need to use condoms or avoid sex for seven days while the hormone builds up in the uterine lining. The same seven-day rule applies if you’re more than 21 days postpartum and your period hasn’t returned, or if the IUD is placed sometime after (rather than immediately following) an abortion. Having unprotected sex during that seven-day window is one of the few timing-related scenarios where a hormonal IUD hasn’t yet reached full effectiveness.