An Intrauterine Device (IUD) offers highly effective contraception, providing long-term birth control. While IUDs are among the most effective reversible birth control methods, no contraceptive offers 100% effectiveness. The rare possibility of pregnancy is a common question for IUD users. Understanding how these T-shaped devices work and factors influencing their effectiveness can clarify conception chances.
How IUDs Work
IUDs prevent pregnancy through different mechanisms, depending on whether they are hormonal or copper-based. Hormonal IUDs (e.g., Mirena, Kyleena, Liletta, Skyla) release a synthetic progestin hormone, similar to natural progesterone. This progestin primarily thickens cervical mucus, forming a barrier that blocks sperm from reaching an egg. It also thins the lining of the uterus, making it difficult for a fertilized egg to implant. In some individuals, hormonal IUDs may also partly suppress ovulation, meaning an egg is not released from the ovary.
Copper IUDs, like Paragard, do not use hormones. Instead, they are wrapped in a small amount of copper, which creates an inflammatory reaction within the uterus. This environment is toxic to sperm, impairing their ability to move and reach an egg. The copper also changes the uterine lining, making it unfavorable for implantation. Both types of IUDs work by altering the uterine environment to prevent conception.
Actual Pregnancy Rates
IUDs are considered one of the most effective forms of reversible birth control, with failure rates typically less than 1%. For hormonal IUDs, the failure rate is low, ranging from approximately 0.02% to 0.04% per year, meaning about one to two pregnancies per 5,000 users. Copper IUDs also boast high effectiveness, with a failure rate around 0.8% annually.
Comparing IUDs to other birth control options, they are as effective as permanent surgical sterilization and more effective than birth control pills, patches, or rings. The high effectiveness of IUDs is largely due to the elimination of user error, as they do not require daily attention or correct usage during sexual activity. Once inserted, an IUD provides continuous protection against pregnancy for several years, depending on the specific type.
Why IUDs Might Fail
While IUDs are highly effective, rare circumstances can lead to failure. One primary reason is IUD expulsion, where the device partially or completely moves out of the uterus. Expulsion rates vary, with some studies reporting rates between 3% and 11%. A dislodged IUD is no longer fully effective at preventing pregnancy.
Another uncommon cause of failure is uterine perforation, which occurs when the IUD punctures the uterine wall during insertion. This complication is rare, estimated to occur in about 1 to 2 per 1,000 insertions. Perforation can lead to the IUD moving outside the uterus, reducing its effectiveness. In some instances, the IUD may be incorrectly positioned within the uterus, which can also diminish its contraceptive efficacy.
Pregnancy with an IUD
If pregnancy occurs while an IUD is in place, immediate medical attention is necessary. There is an increased risk of complications, including ectopic pregnancy, where the fertilized egg implants outside the uterus (usually in a fallopian tube). While IUDs significantly reduce the overall risk of pregnancy, if it occurs, it is more likely to be ectopic than if no birth control was used.
Continuing a pregnancy with an IUD in the uterus also carries elevated risks of miscarriage, preterm delivery, and infection. If the IUD strings are visible, healthcare providers generally recommend removing the device early in pregnancy to reduce these risks. Removing the IUD can lower the chance of pregnancy loss compared to leaving it in place. However, if the IUD cannot be safely removed or its strings are not visible, it may be left in, and the pregnancy will be closely monitored.