An IUD is a small, T-shaped device placed inside the uterus that prevents pregnancy for anywhere from 3 to 12 years, depending on the type. It works with less than a 1% failure rate for both typical and perfect use, making it one of the most effective reversible contraceptives available. There are two types: hormonal IUDs, which release a small amount of a synthetic hormone, and copper IUDs, which use copper ions to create an environment hostile to sperm.
How Hormonal IUDs Prevent Pregnancy
Hormonal IUDs release a tiny, steady dose of levonorgestrel, a synthetic form of progesterone, directly into the uterus. The highest-dose devices release about 20 micrograms per day in their first year, tapering to roughly 12 micrograms per day by the end of their lifespan. That local delivery is key: hormone levels inside the uterus are about 1,000 times higher than what reaches your bloodstream, which is why hormonal IUDs cause far fewer body-wide side effects than the pill or the shot.
This concentrated hormone does three things at once. First, it thickens cervical mucus so significantly that sperm can’t pass through. Studies scoring mucus quality on a standardized scale found that IUD users consistently fell below the threshold needed for sperm to penetrate. Second, it thins the uterine lining, making it inhospitable for a fertilized egg. The hormone essentially blocks the lining’s ability to respond to the body’s natural estrogen, suppressing its growth and sometimes reducing it enough to lighten or stop periods entirely. Third, in some users, it partially suppresses ovulation, though this isn’t the primary mechanism and varies from person to person.
How the Copper IUD Works
The copper IUD (sold as Paragard) contains no hormones. Instead, a copper wire is wrapped around the plastic frame. Once inside the uterus, the copper releases ions that are toxic to sperm. In lab studies, a copper surface area of just 175 square millimeters reduced sperm motility by 99% within two hours. After eight hours, no motile sperm remained. The Paragard device has a copper surface area of 380 square millimeters, well above that threshold.
Copper ions also trigger a mild inflammatory response in the uterine lining that further disrupts sperm function and makes implantation unlikely. Because there’s no hormone involved, the copper IUD won’t change your natural cycle, though periods can become heavier or crampier, especially in the first few months.
Types of IUDs and How Long They Last
Five IUDs are available in the U.S., and they differ in hormone dose, physical size, and approved duration.
- Mirena: 52 mg hormone reservoir, 32mm x 32mm frame, lasts up to 8 years.
- Liletta: 52 mg hormone reservoir, 32mm x 32mm frame, lasts up to 8 years.
- Kyleena: 19.5 mg hormone reservoir, 28mm x 30mm frame, lasts up to 5 years.
- Skyla: 13.5 mg hormone reservoir, 28mm x 30mm frame, lasts up to 3 years.
- Paragard: Copper, no hormones, 32mm x 36mm frame, lasts up to 12 years.
Kyleena and Skyla are physically smaller, which can make insertion more comfortable for people with a smaller uterus or who haven’t been pregnant. The lower hormone dose in these devices means lighter periods are still common, but full cessation of periods is less likely than with Mirena or Liletta.
What Insertion Feels Like
Getting an IUD placed takes about five minutes. A clinician uses a speculum, measures the depth of your uterus with a thin instrument called a sound, then slides the IUD through the cervix using an inserter tube. The arms of the T fold down for insertion and spring open once inside.
Pain varies widely. Some people feel only mild cramping; others describe a sharp, intense but brief pain as the device passes through the cervical opening. The American College of Obstetricians and Gynecologists now recommends that clinicians offer local anesthetics for every IUD insertion, including lidocaine spray, numbing cream, or a paracervical block (an injection that numbs the cervix). You can also ask about sedation options if you’re concerned about pain. The important thing is that pain management should be a conversation before the procedure, not something you have to request after the fact.
Cramping and spotting are normal for a few days afterward. Most people can return to regular activities the same day.
How Effective IUDs Are
IUDs, along with the arm implant, are the most effective reversible contraceptives. Both hormonal and copper IUDs have a failure rate below 1% in typical use, which is the same as perfect use. That’s because once the device is in place, there’s nothing to remember, no pill to take, no patch to replace. Human error is essentially removed from the equation.
For comparison, the pill has a typical-use failure rate of about 7%, largely because people miss doses or refill prescriptions late. The IUD’s set-it-and-forget-it design is its biggest practical advantage.
Effects on Your Period
Hormonal IUDs typically make periods lighter over time. Within the first year, about 20% of people using Mirena or Liletta stop getting periods altogether. Others experience shorter, lighter bleeding or occasional spotting. Irregular spotting is common in the first three to six months as your body adjusts, but it usually settles down.
The copper IUD tends to work in the opposite direction. Periods often become heavier and crampier, particularly in the first several months. This is the most common reason people have the copper IUD removed early. For people who already have heavy periods, a hormonal IUD is generally a better fit.
Possible Complications
The most common issue is expulsion, where the IUD partially or fully slips out of position. In standard (non-postpartum) insertions, expulsion happens in roughly 2 to 10% of users within the first year. The risk is higher when the device is placed immediately after childbirth; one study found vaginal-delivery postpartum expulsion rates as high as 19%, compared to 7% after cesarean delivery. You can check for proper placement by feeling for the IUD strings at the cervix with your fingers. If the strings feel longer, shorter, or are missing, contact your provider.
Uterine perforation, where the device pokes through the uterine wall, is rare, occurring in roughly 1 in 1,000 insertions. Infection risk is slightly elevated in the first 20 days after placement but returns to baseline after that.
Fertility After Removal
An IUD is fully reversible. Once removed, which takes about 30 seconds in a standard office visit, fertility returns quickly. A large review of nearly 2,400 former IUD users found that 83% became pregnant within the first 12 months of removal. For people under 35, the general expectation is a 20% chance of conceiving each menstrual cycle, with about 60% pregnant by six months and 85 to 90% by one year. These rates are comparable to people who’ve never used any long-term contraception, confirming that IUDs don’t cause lasting changes to fertility.