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. There are two kinds: copper IUDs and hormonal IUDs. They work through different mechanisms, but both are over 99% effective, making them among the most reliable forms of birth control available.
How Copper IUDs Prevent Pregnancy
The copper IUD (sold as Paragard in the U.S.) contains no hormones. Instead, it’s wrapped in a small amount of copper wire that continuously releases copper ions into the uterus. These ions trigger an inflammatory reaction inside the uterine cavity that is toxic to sperm. The copper dissolves into cervical mucus at high enough concentrations to stop sperm from swimming normally, and it damages sperm quality and survival throughout the reproductive tract.
The result is dramatic. Studies comparing sperm recovery in the fallopian tubes found that women with a copper IUD in place had far fewer viable sperm reaching the egg than women without one. Fertilization is effectively blocked before it can happen. The copper IUD does not interrupt an existing pregnancy; it works by preventing sperm from ever successfully reaching or fertilizing an egg.
Because it’s hormone-free, the copper IUD doesn’t interfere with your natural menstrual cycle or ovulation. You continue to ovulate each month as usual. The tradeoff is that periods can become heavier, longer, or crampier, particularly in the first few months. This tends to improve over time for most people, but if you already deal with heavy periods, a copper IUD may not be the best fit.
How Hormonal IUDs Prevent Pregnancy
Hormonal IUDs release a small, steady amount of a synthetic progestin directly into the uterus. Four brands are currently available in the U.S.: Mirena, Liletta, Kyleena, and Skyla. They prevent pregnancy through three overlapping mechanisms.
First, they thicken the mucus at the cervix into a dense barrier that sperm can’t easily penetrate. Second, they thin the uterine lining, making it inhospitable. Third, the local hormone environment impairs sperm movement and survival inside the uterus. Together, these effects make it extremely unlikely that sperm will reach an egg, and even less likely that a fertilized egg could implant.
One common question is whether hormonal IUDs stop ovulation entirely. They can suppress ovulation during the first year, but after that, most cycles become ovulatory again. Research shows that about 85% of cycles are ovulatory in long-term hormonal IUD users, the same rate seen in copper IUD users. So while hormonal IUDs may occasionally prevent ovulation, that’s not their primary way of working. The heavy lifting is done locally, right in the uterus and cervix.
How Effective IUDs Actually Are
IUDs are among the most effective contraceptive methods because they require virtually no user effort once placed. The copper IUD has a typical-use failure rate of 0.8% in the first year. Hormonal IUDs are even more effective, with a failure rate of just 0.2%. For comparison, the pill, patch, and ring each have a typical-use failure rate of about 8%, and male condoms fail about 15% of the time with typical use.
The reason IUDs perform so well is that “typical use” and “perfect use” are nearly identical. You can’t forget to take an IUD or use it incorrectly. Once it’s in, it works continuously until it’s removed or expires. This puts IUDs in the same reliability tier as surgical sterilization and contraceptive implants.
How Long Each Type Lasts
The five FDA-approved IUDs have different lifespans:
- Paragard (copper): up to 12 years
- Mirena (hormonal): up to 8 years
- Liletta (hormonal): up to 8 years
- Kyleena (hormonal): up to 5 years
- Skyla (hormonal): up to 3 years
Higher-dose hormonal IUDs like Mirena and Liletta are more likely to make your period stop entirely. Lower-dose options like Kyleena and Skyla tend to make periods lighter without eliminating them. All hormonal IUDs can cause irregular spotting during the first several months.
What Insertion Feels Like
Getting an IUD placed takes only a few minutes in a clinic. Your provider will first do a quick pelvic exam to check the position of your uterus, then use a speculum (like during a Pap smear) to access the cervix. A thin instrument measures the depth of your uterine cavity, which is typically 6 to 9 centimeters. The IUD is then guided through the cervix and into the uterus using a narrow inserter tube. Once placed, the strings are trimmed to about 3 to 4 centimeters so they extend slightly past the cervix for future removal.
Most people feel a sharp cramp during the measurement step and again when the device is placed. The intensity varies widely. Some describe it as a brief, strong pinch; others find it more painful. The cramping usually fades within minutes, though mild cramping and spotting can continue for a few days. Your provider may suggest taking a pain reliever beforehand, and some clinics offer a local anesthetic applied to the cervix. A follow-up visit is typically scheduled 4 to 6 weeks later to confirm the IUD is still in the right position.
Copper IUDs 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 contraceptive pills, and it then continues working as long-term birth control for up to 12 years. This option is worth knowing about, though it does require a clinic visit for placement within that 5-day window.
Risks and Side Effects
Serious complications from IUDs are uncommon. The two main physical risks are expulsion (the device partially or fully slipping out of place) and perforation (the device pushing into or through the uterine wall).
Expulsion happens in about 2.3% of users within the first year and roughly 4.6% over five years. You may notice the strings feel longer than usual, feel the hard plastic of the device at your cervix, or experience unexpected cramping or bleeding. The risk is higher if the IUD is placed very soon after childbirth: about 10.7% over five years for insertions within the first three days postpartum, compared to around 3.2% when placed more than six weeks after delivery.
Perforation is rarer. The overall five-year risk is about 0.6%, and complete perforation (the device passing fully through the uterine wall) occurs in roughly 0.05% of non-postpartum insertions. The risk is higher for IUDs placed in the early postpartum period, particularly while breastfeeding. If perforation occurs, the device typically needs to be removed through a minor procedure.
IUDs do not increase the long-term risk of pelvic infections. There is a small, temporary increase in infection risk in the first few weeks after insertion, related to the placement procedure itself rather than the device.
Fertility After Removal
Fertility returns quickly after an IUD is removed, regardless of type. In one study comparing former IUD users to women using other contraceptive methods, 81% of IUD users became pregnant within 12 months of trying, with no statistical difference in time to pregnancy between the two groups. There is no evidence that either copper or hormonal IUDs cause any lasting delay in fertility. Once the device is out, its contraceptive effects stop immediately.