IUDs are among the most effective forms of birth control available. The hormonal IUD (Mirena) has a first-year failure rate of just 0.2%, while the copper IUD (Paragard) comes in at 0.8%. For comparison, the birth control pill fails about 9% of the time with typical use. That massive gap exists because an IUD works on its own once placed, removing the daily human element that leads to missed doses and inconsistent timing.
Typical Use vs. Perfect Use
Most contraceptives have two effectiveness numbers: perfect use (following every instruction flawlessly) and typical use (how real people actually use them). The pill, for instance, drops from 99.7% effective with perfect use to 91% with typical use, because people forget pills, take them late, or refill prescriptions a day too late. IUDs nearly eliminate that gap. The hormonal IUD is 99.8% effective under both perfect and typical conditions. The copper IUD is 99.4% with perfect use and 99.2% with typical use. Once it’s in, there’s nothing to remember, swallow, or replace for years.
How Each Type Prevents Pregnancy
Copper and hormonal IUDs work through different biological pathways, though both act primarily by stopping sperm from reaching an egg.
The copper IUD releases copper ions into the uterus, which are toxic to sperm. This creates an environment where sperm can’t survive or move effectively enough to fertilize an egg. The copper also triggers a mild inflammatory response in the uterine lining that further reduces the chance of pregnancy.
Hormonal IUDs release a small amount of progestin directly into the uterus. This thickens cervical mucus into a barrier that blocks sperm, thins the uterine lining, and in some users partially suppresses ovulation. Because the hormone acts locally rather than circulating through the entire body, the dose is far lower than what oral contraceptives deliver.
Long-Term Performance Over Years of Use
IUDs don’t lose much ground over time. The copper IUD, which is FDA-approved for 10 years of use, had a cumulative pregnancy rate of just 1.5 per 100 women over 7 years in large studies. A World Health Organization trial followed copper IUD users for 12 years and found a cumulative pregnancy rate of 2.2 per 100 women over that entire span. That means roughly 98 out of 100 women using a copper IUD for over a decade never experienced an unintended pregnancy.
Hormonal IUDs come in several versions with different approved durations. Mirena is approved for 8 years, Kyleena for 5 years, and Skyla for 3 years. All maintain over 99% effectiveness throughout their approved lifespan. The differences in duration relate to the amount of hormone each device contains and how long it can sustain adequate release.
Effectiveness Across Body Types
Unlike some hormonal methods, IUD effectiveness holds steady regardless of body weight. Because both copper and hormonal IUDs act directly inside the uterus rather than relying on hormones traveling through the bloodstream, a higher body mass doesn’t dilute their effect. Research comparing IUD outcomes across normal weight, overweight, and obese users found no statistically significant difference in 12-month continuation rates between groups. No contraceptive failures occurred in any weight category in that study. This makes IUDs a particularly reliable option for people who weigh more, since some other methods (like the patch and certain emergency contraception pills) can become less effective at higher body weights.
The Copper IUD as Emergency Contraception
The copper IUD doubles as the most effective form of emergency contraception available. When inserted within 120 hours (5 days) of unprotected sex, it prevents pregnancy more than 99% of the time. That’s significantly more effective than emergency contraception pills, and it then continues working as ongoing birth control for up to 10 years. This option is worth knowing about even if you aren’t planning to use an IUD long-term.
What Can Reduce Effectiveness
The main risk factor that undermines IUD protection is expulsion, when the device partially or fully slips out of position. First-year expulsion rates range from 2% to 10%, and the 3-year cumulative expulsion rate is about 10 per 100 users. Expulsion rates are similar for copper and hormonal IUDs.
Age plays a significant role. Teenagers aged 14 to 19 have expulsion rates roughly two to three times higher than older users. Women who have previously given birth also have somewhat higher expulsion rates than those who haven’t, though the overall risk remains relatively low for both groups.
An expelled IUD can’t prevent pregnancy, and partial expulsion can happen without obvious symptoms. This is why periodic string checks (feeling for the thin threads that hang from the cervix) matter. If the strings feel shorter, longer, or disappear entirely, the device may have shifted. Even with expulsion factored in, the risk of losing an IUD is still lower than the risk of discontinuing a shorter-acting method like the pill or the injectable shot.
How Quickly Protection Starts and Stops
The copper IUD begins working immediately after insertion, regardless of where you are in your menstrual cycle. This is part of why it works as emergency contraception. Hormonal IUDs are also effective right away if inserted during the first 7 days of your period. If inserted at any other time in your cycle, you’ll need backup contraception (like condoms) for the first 7 days.
On the other end, fertility returns immediately after removal. There is no washout period or hormonal reset needed. Depending on where you are in your cycle when the IUD comes out, pregnancy is possible before your next period even arrives. About 80% of couples who try to conceive after IUD removal achieve pregnancy within 12 months, which matches the general population’s conception rate. The IUD does not affect long-term fertility.