An intrauterine device (IUD) is a small, T-shaped form of long-acting, reversible contraception placed inside the uterus. This highly effective device works by preventing fertilization or implantation, providing protection for several years depending on the specific type. Because the IUD is a physical object placed within an internal organ, questions about its precise positioning are common. The successful function of the IUD is entirely dependent on its correct placement within the reproductive system.
The Uterus: The IUD’s Resting Place
The uterus is the organ where the IUD resides. This muscular organ contains a hollow internal space called the uterine cavity. The average uterine cavity depth, measured from the opening of the cervix to the top of the uterus, typically falls between 6 and 9 centimeters in length.
The IUD is specifically engineered to sit in the upper region of this cavity, known as the fundus. The fundus is the dome-shaped top part of the uterus. Proper placement means the two horizontal arms of the T-shaped device rest fully extended across this upper space, with the stem pointing down toward the cervix. The device is designed to float freely within the hollow cavity and should not be embedded in the muscular wall of the uterus itself.
The goal of insertion is for the IUD to be positioned “high-fundal,” meaning it is as close to the top of the cavity as possible, ensuring maximum contact with the uterine lining. This placement is necessary for the IUD to be fully effective, whether it is a copper IUD or a hormonal IUD that releases progestin. The IUD must be fully contained within the cavity, and its width is designed to fit the cavity’s natural dimensions, generally between 28 and 32 millimeters wide.
The Insertion Procedure and Depth Measurement
Achieving the correct high-fundal placement requires a precise measurement of the patient’s internal anatomy before the IUD is inserted. Clinicians use a specialized tool called a uterine sound, which is a thin, flexible rod used to measure the depth and determine the direction of the uterine cavity. This sounding procedure measures the exact distance from the cervical opening, through the cervical canal, to the top of the uterus.
The average depth is typically recorded as 6 to 8 centimeters, and this measurement is used to prepare the IUD inserter tube. The inserter, which contains the folded IUD, is calibrated by setting a plastic stopper, or flange, to the measured depth. This ensures that when the clinician advances the inserter through the cervix, the IUD is released precisely at the fundus, avoiding shallow placement and excessive force. After the IUD is deployed, the clinician often confirms the placement by feeling for the firm resistance of the device against the uterine wall or by using an ultrasound.
When Placement Changes: Migration and Expulsion
While the IUD is designed to remain securely in the fundus, its position can occasionally change. These changes fall into two main categories: expulsion and migration. Expulsion occurs when the IUD partially or completely slides out of the uterus, usually dropping through the cervix and into the vagina. This movement is most common within the first year after insertion and can be triggered by uterine contractions, particularly during a menstrual period.
Migration refers to the IUD moving from its optimal position within the uterine cavity. This includes the rare complication of the device moving through the uterine wall, known as uterine perforation. Perforation occurs in about 1 to 2 out of every 1,000 insertions. A perforation can be partial, where the device is partially embedded in the uterine muscle, or complete, where it passes entirely through the wall and into the abdominal cavity. While often attributed to improper placement, it is also possible for an IUD to gradually move through the wall over time, a process sometimes called transmural migration.
How to Check IUD Position at Home
After insertion, the IUD has one or two thin nylon strings that hang a few centimeters out of the cervix into the upper part of the vagina. These strings serve as the primary way for a healthcare provider to remove the device and for the patient to confirm its placement. Checking the IUD’s position involves feeling for these strings, a simple self-check that many clinicians recommend performing monthly, such as after a menstrual period.
To check the strings, a person should wash their hands, squat or sit comfortably, and insert a finger into the vagina until they can feel the firm, rubbery cervix. The strings should feel like pieces of fine, taut thread, similar to dental floss, hanging from the cervical opening. If the strings feel longer or shorter than they did during the previous check, or if the hard plastic of the IUD itself can be felt, this suggests the IUD may have moved out of its proper high-fundal position, and a medical evaluation is needed. If the strings cannot be felt at all, the IUD may have been expelled, or they may have curled up inside the cervix, requiring a clinician to determine its location.