An intrauterine device (IUD) is a small, T-shaped piece of flexible plastic placed inside the uterus. IUDs are considered one of the most reliable methods for preventing pregnancy, offering continuous protection over several years. Because the device works directly within the uterus, it is significantly less susceptible to the human error associated with daily birth control methods. Despite its exceptional effectiveness, the possibility of pregnancy, while extremely remote, does exist. The risk is primarily linked to the device’s function and whether it remains correctly positioned inside the uterine cavity.
Quantifying the Likelihood of Failure
The possibility of pregnancy with an IUD is quantified by extremely low failure rates, typically reported using the Pearl Index. This index calculates the number of pregnancies per 100 woman-years of use. For IUDs, the failure rate under typical use is less than one percent per year, often falling between 0.1% and 0.8% annually. This high level of efficacy places the IUD among the most effective contraceptive options available, comparable to surgical sterilization. The IUD’s success rate stems from its continuous, localized action, eliminating the daily compliance requirements that affect the effectiveness of other methods.
Variations in Device Efficacy
The possibility of pregnancy is influenced by the specific type of IUD used: hormonal or non-hormonal copper. The hormonal IUD releases the progestin levonorgestrel, working primarily by thickening cervical mucus, thinning the uterine lining, and sometimes suppressing ovulation. Hormonal IUDs have demonstrated a failure rate as low as 0.1% to 0.4% per year. The copper IUD works by creating a localized inflammatory reaction toxic to sperm and eggs, preventing fertilization and implantation. Copper IUD failure rates are generally reported to be around 0.6% to 0.8% per year.
Recognizing IUD Displacement or Expulsion
The primary reason an IUD may fail is its displacement or complete expulsion from the uterus. Displacement occurs when the device shifts from its optimal position, compromising its ability to prevent fertilization or implantation. Users should monitor for this by periodically checking the IUD’s strings, which extend into the vagina. If the strings feel noticeably longer or shorter than usual, or cannot be felt at all, the IUD may have moved.
Other physical signs that the IUD is out of place include:
- Sharp or persistent cramping.
- Unusual vaginal bleeding or discharge.
- Pain during sexual intercourse.
- Feeling the hard, plastic tip of the IUD protruding from the cervix (partial expulsion).
If any of these symptoms or changes in string length are noted, a healthcare provider should be contacted immediately. Until the IUD’s correct position is confirmed, a backup barrier method of contraception should be used.
Medical Management of Pregnancy with an IUD
If pregnancy occurs despite an IUD being in place, it requires immediate medical attention. A significant risk is that the pregnancy may be ectopic, meaning it is located outside the uterus, usually in a fallopian tube. Because the IUD is highly effective against intrauterine pregnancy, a higher proportion of resulting pregnancies are ectopic. An ultrasound is necessary to confirm the location of the gestation and the position of the IUD.
If the pregnancy is intrauterine, the IUD should generally be removed as soon as possible, provided the strings are visible and removal is safe. Removing the IUD reduces the risk of complications, including miscarriage, severe infection, and preterm delivery. If the strings are not visible or the device cannot be removed safely, it may be left in place, but this carries an elevated risk of spontaneous abortion and severe uterine infection. Close monitoring is necessary throughout the remainder of the pregnancy.