An intrauterine device, or IUD, is a small, T-shaped form of long-acting reversible contraception inserted directly into the uterus. The device is one of the most effective birth control methods available, offering continuous protection from pregnancy for several years. Its high efficacy is due to its mechanism of action and the elimination of user error. However, no contraceptive method is completely foolproof, and understanding the minute chance of conception is important.
Comparing Failure Rates by IUD Type
The effectiveness of an IUD is consistently high, with both available types demonstrating failure rates of less than one percent per year. This measurement is often quantified using the Pearl Index, which calculates the number of pregnancies per 100 women-years of use. The hormonal IUD, which releases a progestin hormone, is slightly more effective than its non-hormonal counterpart, the copper IUD.
Hormonal IUDs typically show a failure rate as low as 0.06 to 0.4 pregnancies per 100 women in the first year of use, which translates to an effectiveness of over 99.5%. This type primarily works by thickening cervical mucus to block sperm and thinning the uterine lining. The copper IUD, which works by releasing copper ions toxic to sperm and eggs, has a slightly higher, yet still very low, failure rate, often around 0.52 to 0.8 pregnancies per 100 women-years.
This difference in failure rates stems from the distinct mechanisms of action. The copper IUD’s efficacy is based on the localized inflammatory reaction and spermicidal effect created by the copper, which prevents fertilization. The hormonal IUD adds a layer of protection through the systemic and local effects of the progestin. Both types of IUDs belong to a class of contraceptives whose real-world failure rates are nearly identical to their perfect-use failure rates.
Understanding the Real-World Chances of Conception
The primary reason IUDs boast such high effectiveness in real-world use is that they remove the factor of user error. Unlike birth control pills, patches, or condoms, the IUD does not require daily action or correct use at the time of intercourse to be effective. Once a clinician properly inserts the device, the user is continuously protected for the lifespan of the IUD.
The small chance of failure that does exist is often linked to two main factors: improper initial placement and IUD expulsion. Correct insertion by a trained clinician is crucial, as a device that is not seated properly within the uterine cavity may not function as intended.
Even with perfect placement, there is a low risk, estimated at about 4% within the first year, that the uterus will partially or completely push the device out, known as expulsion. Expulsion can sometimes go unnoticed, especially if the user experiences lighter or absent periods due to a hormonal IUD. The risk of expulsion is slightly higher for the copper IUD compared to the hormonal IUD, with rates around 6% in the first year for the copper type. Importantly, common misconceptions about factors like weight or the use of antibiotics have been shown not to affect the IUD’s highly localized contraceptive action.
Risks When Pregnancy Happens With an IUD In Place
When the rare event of conception occurs with an IUD still in the uterus, the immediate concern is determining the location of the pregnancy. An IUD does not increase the overall risk of an ectopic pregnancy, but if a pregnancy does occur, there is a significantly higher likelihood it will be ectopic, meaning it implants outside the main cavity of the uterus. This is a medical emergency that requires immediate intervention.
For pregnancies confirmed to be ectopic, the risk is approximately 15% for those with a copper IUD and can be as high as 50% for those with a hormonal IUD. This high percentage is because the IUD is so effective at preventing intrauterine implantation that any pregnancy that manages to occur is more likely to have implanted elsewhere. Therefore, any confirmed pregnancy with an IUD in place requires an immediate ultrasound to verify the gestational sac’s location.
If the pregnancy is confirmed to be intrauterine, the presence of the IUD still presents risks to the developing fetus and the pregnancy. If the IUD is left in place, the risk of a miscarriage is approximately 55%, with an increased danger of septic miscarriage, which is life-threatening. If the IUD strings are visible and the device can be safely removed by a healthcare provider early in the pregnancy, the miscarriage risk drops substantially to about 20%. The decision to remove or retain the device must be made through immediate consultation with a medical professional, weighing the small risk of miscarriage from the removal procedure against the significantly higher risks associated with a retained device.