No IUD is 100% effective, but it comes very close. Fewer than 1 out of 100 people with an IUD will become pregnant in the first year of use, making it one of the most reliable reversible contraceptives available. That effectiveness holds for both copper and hormonal types, and because an IUD works on its own once placed, there’s virtually no difference between “perfect use” and “typical use” failure rates.
How Effective IUDs Actually Are
The failure rate for IUDs in the first year is less than 1%, which puts them in the same tier as the contraceptive implant. For context, that’s more effective than the pill (which fails about 7% of the time with typical use), the patch, the ring, and even condoms. The reason the gap is so large is simple: an IUD doesn’t require you to remember anything, take anything on schedule, or use it correctly each time. Once it’s in place, it’s working.
A large study of over 83,000 people found that hormonal IUDs were actually more effective than tubal ligation (surgical sterilization) at preventing pregnancy, while copper IUDs were equally effective. That finding surprised many clinicians, since sterilization is permanent and requires surgery. The real-world pregnancy rates were higher than the commonly cited “1 in 1,000” figure, though. Researchers found that about 2.4% of hormonal IUD users and 3% of copper IUD users became pregnant over the study period, compared to 2.6% of those who had tubal ligations. These numbers reflect a broader population that includes people whose IUDs may have shifted or been expelled without their knowledge.
How Each Type Prevents Pregnancy
Copper and hormonal IUDs work through different mechanisms, but both act locally inside the uterus.
The copper IUD releases copper ions that are toxic to sperm. Lab studies show that the amount of copper surface area in a standard IUD reduces sperm motility by 99% within two hours. By eight hours, no motile sperm remain. This means sperm are effectively immobilized before they can reach an egg. The copper IUD contains no hormones at all.
Hormonal IUDs release a small amount of a progestin that acts primarily on the cervix. It thickens cervical mucus to the point where sperm simply cannot pass through. This effect is local, meaning the hormone concentrations in the rest of your body stay relatively low. Hormonal IUDs do not work by disrupting an existing pregnancy.
When Protection Starts
The two types differ here. A copper IUD is effective immediately after insertion, no matter where you are in your menstrual cycle. You don’t need backup contraception.
A hormonal IUD is immediately effective only if it’s placed within the first seven days of your period. If it’s placed at any other time in your cycle, you need to use condoms or abstain for seven days while it takes effect.
What Can Reduce Effectiveness
The main reason an IUD fails is displacement or expulsion, meaning the device shifts out of its correct position in the uterus or comes out entirely. When the IUD is properly positioned, the failure rate is extremely low. When it moves, protection drops.
Expulsion rates vary depending on when the IUD is placed. For IUDs inserted right after delivery, expulsion rates in studies have ranged from about 10% to 28%, with lower rates after cesarean delivery (7%) compared to vaginal delivery (19%). IUDs placed outside of the postpartum period have considerably lower expulsion rates, but the risk is never zero.
Signs that your IUD may have moved include new or worsening cramping, unusual bleeding (especially in the weeks after insertion), feeling the hard plastic of the IUD at the opening of the cervix, or noticing that the strings feel longer or shorter than usual. If the strings disappear entirely, that can mean the IUD has shifted upward or been expelled without you noticing. An ultrasound, particularly a 3D ultrasound, is the most reliable way to confirm whether an IUD is still in the right place.
Does Body Weight Affect How Well It Works?
For most people, no. Systematic reviews have found that both copper and hormonal IUDs maintain their effectiveness across all weight categories. Even though blood levels of the hormone from a hormonal IUD are somewhat lower in people with higher body weight, the contraceptive effect is preserved because the hormone acts locally in the uterus rather than through the bloodstream. People with a BMI of 40 or above may have slightly higher rates of IUD expulsion, but this hasn’t translated into higher pregnancy rates in the available research.
This is a meaningful advantage over some other hormonal methods, where higher body weight can reduce how well they work.
What’s Available in the U.S.
Four IUDs are currently available: one copper IUD and three hormonal IUDs with different hormone doses. The hormonal versions contain either 13.5 mg, 19.5 mg, or 52 mg of progestin, which affects both how long they last and how much they change your periods. The higher-dose hormonal IUDs tend to reduce menstrual bleeding significantly, while the copper IUD can make periods heavier, especially in the first few months.
All four have failure rates below 1% in the first year with typical use, making the choice between them more about side effects, duration, and personal preference than about which one prevents pregnancy better.