An IUD prevents pregnancy primarily by stopping sperm from reaching an egg. It’s a small, T-shaped device placed inside the uterus, and depending on the type, it uses either copper or a slow release of hormones to create an environment where fertilization can’t happen. With over 99% effectiveness in both typical and perfect use, IUDs are among the most reliable contraceptives available.
There are two fundamentally different types, and they work through different mechanisms.
How the Copper IUD Prevents Pregnancy
The copper IUD (sold as ParaGard) contains no hormones. Instead, a thin copper wire wrapped around the device releases copper ions into the uterus, creating a toxic environment for sperm. Research published in Fertility and Sterility found that copper reduces sperm motility by 99% within just two hours of exposure, and after eight hours, no motile sperm remained. Sperm that can’t swim can’t reach an egg.
Copper also triggers a mild inflammatory response in the uterine lining. This inflammation makes it much harder for a fertilized egg to implant, adding a secondary layer of protection. The copper IUD is effective immediately after insertion, so no backup contraception is needed.
How Hormonal IUDs Prevent Pregnancy
Hormonal IUDs release small amounts of a synthetic progesterone called levonorgestrel directly into the uterus. This creates multiple barriers to pregnancy at once.
The most important effect is on cervical mucus. The hormone thickens the mucus at the entrance to the uterus, forming a barrier that sperm struggle to penetrate. At the same time, it thins the uterine lining so there’s less tissue for a fertilized egg to attach to. Hormonal IUDs also partially suppress ovulation, meaning your ovaries may not release an egg every cycle, though this isn’t the primary way they work.
If your hormonal IUD is placed within the first seven days of your period, it’s effective right away. If it’s placed at any other time in your cycle, you’ll need to use condoms or avoid intercourse for seven days while the hormones take effect.
What Insertion Feels Like
The procedure typically takes less than five minutes. Your provider first performs a manual exam to check the position and size of your uterus, then uses a speculum (like during a Pap smear) to access the cervix. A small instrument steadies the cervix, and a thin rod measures the depth of your uterus to ensure the IUD will fit properly. The IUD is then placed through the cervix into the uterus using a narrow insertion tube.
Most people feel strong cramping during the measurement and insertion steps. The intensity varies widely. Some describe it as a sharp pinch that passes quickly, while others find it significantly painful. Cramping and spotting for a few days afterward are normal.
How Each Type Affects Your Period
The two types push your period in opposite directions. Hormonal IUDs thin the uterine lining, so there’s simply less tissue to shed each month. Periods typically become lighter and shorter. About 2 in 10 people with a higher-dose hormonal IUD like Mirena or Liletta stop getting periods entirely within the first year. This isn’t harmful; it just means the lining is too thin to produce noticeable bleeding.
Copper IUDs tend to do the opposite. Because copper triggers inflammation in the uterine lining, many people experience heavier and longer periods, especially during the first year. Your body doesn’t always adjust, and heavier bleeding can persist for the life of the device. You will continue to have regular periods with a copper IUD since it doesn’t affect your hormones or ovulation cycle.
How Long Each IUD Lasts
Different brands are approved for different durations:
- ParaGard (copper): FDA-approved for 10 years, with studies supporting use up to 12 years
- Mirena: approved for up to 6 years, with evidence supporting up to 7
- Liletta: approved for up to 6 years, with evidence supporting up to 7
- Kyleena: approved for 5 years
- Skyla: approved for 3 years
The differences in duration mostly come down to hormone dose. Mirena and Liletta contain the highest amount of levonorgestrel (52 mg), while Kyleena has 19.5 mg and Skyla has 13.5 mg. Lower-dose devices run out of hormone sooner. The copper IUD lasts longest because copper doesn’t deplete the way a hormone reservoir does.
All IUDs can be removed at any time if you want to become pregnant or switch methods. Fertility returns quickly after removal.
Risks: Expulsion and Perforation
The two main physical risks of an IUD are expulsion (the device slipping out of place) and perforation (the device pushing through the uterine wall). Both are uncommon.
Expulsion happens in about 2.3% of users within the first year, regardless of whether the IUD is hormonal or copper. You might notice the IUD’s strings feel longer or different, or you might feel the hard plastic of the device at your cervix. Partial expulsions can reduce effectiveness, so checking your strings periodically helps catch this early.
Perforation is rare. Within the first year, it occurs in roughly 0.2% of hormonal IUD users and 0.16% of copper IUD users. Perforation is most likely to happen during insertion itself and may require a minor procedure to retrieve the device. Risk factors include recent childbirth and breastfeeding, both of which soften uterine tissue.
Hormonal vs. Copper: Choosing by Mechanism
The copper IUD is the only highly effective non-hormonal option for long-term contraception. It’s also the only IUD that works as emergency contraception if inserted within five days of unprotected sex. People who want to avoid synthetic hormones entirely, or who have had side effects from hormonal methods, often choose it despite the heavier periods.
Hormonal IUDs deliver their progestin locally to the uterus rather than systemically through your bloodstream, which means the hormonal effects are more contained than with pills or injections. For people dealing with heavy or painful periods, a hormonal IUD can serve double duty: preventing pregnancy while significantly reducing menstrual bleeding. The trade-off is that some people experience hormonal side effects like acne, headaches, or mood changes, though these are generally milder than with other hormonal methods.
Both types are over 99% effective, so the choice usually comes down to how you want your period to change, how long you want the device to last, and whether you’re comfortable with hormones.