IUDs are more than 99% effective at preventing pregnancy, making them one of the most reliable forms of birth control available. Fewer than 1 out of 100 people who use an IUD will get pregnant in a given year. That effectiveness holds for both hormonal and copper versions, and it stays consistent year after year without requiring you to do anything after insertion.
Why IUDs Outperform Most Other Methods
On paper, many contraceptives look comparable. The pill, for instance, is also over 99% effective when used exactly as directed. But “exactly as directed” is the catch. In real life, the pill’s effectiveness drops to around 95% because people miss doses, take them at inconsistent times, or have interactions with other medications. That 4-to-5 percentage point gap translates to a significant difference in unintended pregnancies across a population.
IUDs sidestep this problem entirely. Once one is placed, there’s nothing to remember, nothing to refill, and no way to use it incorrectly. That’s why researchers classify IUDs as long-acting reversible contraceptives, a category that consistently shows the lowest real-world failure rates of any reversible method. The effectiveness you see in clinical studies is essentially the effectiveness you get in everyday life.
How Each Type Prevents Pregnancy
Hormonal IUDs release a small amount of progestin directly into the uterus. This works through three overlapping mechanisms: it thins the uterine lining so a fertilized egg is unlikely to implant, it thickens cervical mucus into a barrier that blocks sperm from entering, and it impairs sperm movement and survival inside the uterus. Some hormonal IUDs also suppress ovulation in a portion of cycles, though this isn’t the primary way they work.
The copper IUD contains no hormones. Instead, copper ions create an environment that is toxic to sperm, disabling them before they can reach an egg. This reaction begins immediately after placement, which is why the copper IUD is the only type that doubles as emergency contraception (more on that below).
How Long Each IUD Lasts
Not all IUDs are approved for the same duration. The differences matter if you’re choosing between them:
- Liletta: FDA-approved for up to 8 years of pregnancy prevention
- Mirena: approved for up to 8 years
- Kyleena: approved for up to 5 years
- Skyla: approved for up to 3 years
- Paragard (copper): approved for up to 10 years
All of them can be removed earlier if you want to become pregnant or switch methods. Fertility typically returns quickly after removal.
When Protection Starts
The copper IUD is effective immediately. No backup method is needed after placement, regardless of where you are in your cycle.
Hormonal IUDs work a bit differently. If yours is placed within the first 7 days of your period, it’s effective right away. If it’s placed at any other point in your cycle, you’ll need to use condoms or avoid intercourse for 7 days while the hormone builds up enough to block sperm reliably. The same 7-day backup window applies if you’re postpartum and your period hasn’t returned, or if you’re switching from another contraceptive method mid-cycle.
The Copper IUD as Emergency Contraception
When inserted within 5 days of unprotected sex, the copper IUD is the most effective form of emergency contraception that exists. A systematic review covering 35 years of data found a failure rate of just 0.09%, meaning 99.9% of people who received one after unprotected intercourse did not become pregnant. That’s far more effective than emergency contraceptive pills, and it has the added benefit of continuing to work as long-term birth control for up to a decade afterward.
What Can Reduce Effectiveness
The main thing that compromises an IUD’s effectiveness is expulsion, which means the device partially or fully slips out of place. Over three years of follow-up in a large study, about 10 out of every 100 IUD users experienced an expulsion. Rates were similar for hormonal and copper types (10.1 and 10.7 per 100 users, respectively). Most expulsions happen in the first few months after insertion, and you can often detect them by checking for the IUD’s strings.
Body weight may also play a role in expulsion risk. Research in adolescents and young adults found that each increase in BMI category was associated with roughly 2.8 to 4.3 times higher odds of the hormonal IUD being expelled within the first year. Importantly, this doesn’t mean the IUD works less well in people with higher body weight. When the device stays in place, it remains fully effective regardless of BMI. The concern is specifically about whether it stays put.
Having never been pregnant does not increase expulsion risk. A large contraceptive study that included over 2,100 people who had never given birth found no elevated rate of expulsion compared to those who had.
IUDs and Ectopic Pregnancy
A common concern is whether IUDs increase the risk of ectopic pregnancy, where a fertilized egg implants outside the uterus. They don’t. Because IUDs are so effective at preventing pregnancy overall, the absolute risk of any pregnancy, including ectopic, is lower with an IUD than without contraception. However, in the rare event that a pregnancy does occur with an IUD in place, it’s more likely to be ectopic than it would be in a typical pregnancy. This is a statistical quirk rather than a caused risk: the IUD is better at preventing normal uterine pregnancies than ectopic ones, so ectopic pregnancies make up a larger share of the small number of failures.
How IUDs Compare to Other Long-Acting Methods
The contraceptive implant, a small rod placed in the upper arm, has a similarly high effectiveness rate and is also classified as a long-acting method. Both the implant and IUDs sit in a tier well above short-acting options like the pill, patch, ring, and injectable when real-world use is factored in. The choice between an IUD and an implant often comes down to side effect preferences, how long you want protection, and whether you want a hormonal or non-hormonal option. Only the copper IUD offers completely hormone-free contraception in this category.
For people whose primary goal is reliable, low-maintenance pregnancy prevention, IUDs consistently rank at the top. The combination of a failure rate under 1%, years of continuous protection, and no daily action required is difficult for any other reversible method to match.