How Does An Iud Work? Guide

An intrauterine device (IUD) is a small, T-shaped piece of plastic placed inside the uterus to prevent pregnancy. It works by stopping sperm from reaching and fertilizing an egg. IUDs are one of the most effective forms of birth control available, with a failure rate of less than 1% per year.

How does an IUD prevent pregnancy?

There are two main types of IUDs: copper and hormonal. They prevent pregnancy through different mechanisms, but both work before fertilization occurs.

A copper IUD releases copper ions into the uterus. Copper is toxic to sperm. It changes the fluids inside the uterus and fallopian tubes, which makes it difficult for sperm to move and survive. The copper also creates an inflammatory reaction in the uterine lining that is hostile to sperm and eggs. This prevents fertilization. The copper IUD does not contain hormones.

A hormonal IUD releases a low dose of the progestin hormone levonorgestrel. This hormone thickens the mucus at the cervix, creating a plug that blocks sperm from entering the uterus. It also thins the uterine lining (endometrium), making it less suitable for implantation. In some users, hormonal IUDs may also prevent ovulation, but this is not their primary mechanism. The main action is stopping sperm from reaching the egg.

Neither type of IUD works by causing an abortion. Research consistently shows that IUDs prevent fertilization, not implantation of an already fertilized egg.

What are the types of IUDs available?

In the United States, there are five IUD brands approved by the Food and Drug Administration. They fall into two categories.

Copper IUD: Paragard is the only copper IUD available. It contains no hormones and can stay in place for up to 12 years. It is effective immediately after insertion.

Hormonal IUDs: These all contain levonorgestrel but at different doses and with different approved lifespans.

• Mirena and Liletta: Approved for up to 8 years. They release a higher dose of hormone initially and lower over time.
• Kyleena: Approved for up to 5 years. It has a slightly lower hormone dose than Mirena.
• Skyla: Approved for up to 3 years. It is the smallest hormonal IUD and has the lowest hormone dose.

The choice between copper and hormonal IUDs often depends on your menstrual cycle. Copper IUDs can cause heavier periods and more cramping. Hormonal IUDs typically reduce bleeding and may stop periods altogether after a few months.

How is an IUD inserted and removed?

A healthcare provider inserts an IUD during an office visit. The procedure usually takes less than five minutes. The provider uses a speculum to see the cervix, cleans the cervix, and then gently inserts the IUD through the cervical opening into the uterus. A thin plastic string hangs down from the IUD through the cervix into the vagina. The provider trims the string so it does not hang outside the body.

Insertion can cause cramping or discomfort. Many providers recommend taking ibuprofen beforehand. Some women feel lightheaded during or right after insertion, but this usually passes quickly.

Removal is simpler. The provider pulls the string gently, which causes the IUD arms to fold up and slide out. This takes a few seconds and often causes a brief cramp. Fertility returns immediately after removal for both types.

You can check your IUD strings yourself by washing your hands and reaching into the vagina. You should be able to feel the strings near the cervix. If you cannot feel the strings or feel the hard plastic of the IUD itself, contact your provider to confirm the IUD is still in place.

How effective is an IUD compared to other birth control?

IUDs have a failure rate of less than 1% per year with typical use. This means fewer than 1 out of 100 women will become pregnant in the first year of using an IUD. This makes IUDs as effective as female sterilization and more effective than birth control pills, patches, rings, and condoms.

Birth control pills have a typical-use failure rate of about 7 out of 100 women per year. Condoms have a typical-use failure rate of about 13 out of 100 per year. The difference is that IUDs cannot be used incorrectly. Pills can be missed, and condoms can break or slip. Once an IUD is in place, it works continuously for years without any daily action on your part.

Long-acting reversible contraceptives like IUDs and the implant are considered first-line options by major medical organizations including the American College of Obstetricians and Gynecologists.

What are the side effects and risks of an IUD?

Side effects vary by type and person. Most side effects are temporary and not dangerous.

Common side effects with copper IUDs: Heavier periods, longer periods, spotting between periods, and cramping during the first few months. These often improve over time.

Common side effects with hormonal IUDs: Irregular bleeding or spotting in the first 3–6 months. Many women eventually have lighter periods or no periods at all. Some experience breast tenderness, mood changes, acne, or headaches. These side effects are usually mild and improve after the first few months.

Risks and complications: Serious problems are rare.

• Perforation: The IUD can poke through the wall of the uterus during insertion. This occurs in fewer than 1 in 1,000 insertions. It usually heals on its own but may require surgery if the IUD moves into the abdomen.
• Expulsion: The IUD can partially or completely slide out of the uterus. This happens in about 2–10% of users, most often in the first year. Expulsion is more likely in women under 20 and those who have never been pregnant.
• Infection: The risk of pelvic infection is highest in the first 20 days after insertion. After that, the risk is very low and not higher than the general population. Screening for sexually transmitted infections (STIs) before insertion reduces this risk.
• Pregnancy with an IUD in place: This is very rare. If pregnancy occurs, there is a higher risk of ectopic pregnancy, miscarriage, and preterm delivery. If a pregnancy is confirmed with an IUD in place, the IUD should be removed as early as possible if the pregnancy is desired and the strings are visible.

When deciding whether to get an IUD, discuss your medical history with a provider. Most women can safely use an IUD, including those who have never been pregnant.

Can an IUD cause infertility?

No. IUDs do not cause infertility. Fertility returns to normal immediately after the IUD is removed. Studies have found no difference in pregnancy rates between women who have used an IUD and those who have used other methods of birth control.

The confusion about IUDs and infertility dates back to the Dalkon Shield, an unsafe IUD from the 1970s that was linked to pelvic infections and infertility. Modern IUDs are completely different and do not carry that risk. However, having an untreated sexually transmitted infection at the time of insertion can lead to pelvic inflammatory disease, which can damage fertility. That is why providers screen for STIs before placing an IUD.

If you have a current STI or are at high risk, the IUD can be placed after treatment or when the infection is ruled out. For most women, the IUD is a safe and reversible choice that does not affect future fertility.