An intrauterine device (IUD) is a small, T-shaped contraceptive device placed inside the uterus. It is one of the most effective forms of long-acting reversible contraception available today, with an efficacy rate over 99%. The IUD provides continuous birth control, eliminating the need for daily or monthly routines associated with methods like the pill or patches.
The Precise Location of the IUD
The IUD’s intended and most effective placement is high up within the uterine cavity, specifically resting against the fundus of the uterus. The fundus is the uppermost, dome-shaped portion of the muscular organ. Correct positioning requires the IUD’s two horizontal arms to be fully expanded, extending laterally toward the uterine cornua, where the fallopian tubes connect.
This precise location is necessary because it places the device in the area where fertilization most commonly occurs and where implantation would first be attempted. The T-shaped IUD is designed to fit the uterine cavity, though its width is often slightly larger than the average fundal width.
The slight size mismatch between the device and the uterine cavity helps the IUD remain secure within the muscle walls. If the device is positioned more than two centimeters below the fundus, it is considered low-lying or displaced, which may reduce its contraceptive effectiveness. A correctly positioned IUD has its vertical stem extending straight down toward the cervix, with the arms fully securing the device against the top of the cavity.
The Insertion Procedure
IUD placement begins with a healthcare provider performing a pelvic examination to determine the size, shape, and direction of the uterus. This assessment ensures the device will fit properly and helps plan the insertion path. A sterile instrument called a uterine sound is then gently passed through the cervix to accurately measure the depth of the uterine cavity.
This measurement, which typically falls between six and nine centimeters, allows the provider to set a depth gauge on the IUD inserter tube. The T-shaped IUD is kept folded flat inside this narrow tube for easy passage through the cervix. The provider guides the inserter tube into the uterus until it reaches the pre-measured depth near the fundus.
Once the inserter is in the optimal position, the provider releases the IUD, allowing the horizontal arms to spring open into the T-shape inside the uterine cavity. The inserter tube is then withdrawn, leaving the device securely in place at the top of the uterus. Finally, the thin retrieval threads attached to the IUD are trimmed so that about two to three centimeters hang down into the vagina.
How the IUD Stays Secure and Functions
The IUD remains fixed in its fundal position primarily due to the natural muscular environment of the uterus. The horizontal arms of the T-shape press gently against the muscular walls, creating a subtle tension that holds the device in place. The IUD’s design is engineered to resist the uterus’s natural tendency to expel any foreign object.
Both copper and hormonal IUDs prevent pregnancy by altering the environment inside the uterus to be inhospitable to sperm. The copper IUD releases copper ions into the uterine fluid, which are toxic to sperm, impairing their motility and viability before they can reach the egg. This action also induces a localized, non-infectious inflammatory reaction that further prevents fertilization.
The hormonal IUD, in contrast, releases a low, steady dose of progestin, typically levonorgestrel, directly into the uterine cavity. This hormone thickens the cervical mucus, creating a physical barrier that blocks sperm from entering the uterus. Progestin also causes the lining of the uterus to thin, which makes it difficult for a fertilized egg to implant, though the primary action is to prevent fertilization.
Monitoring the IUD Position
After insertion, the patient is advised to periodically check the position of the IUD by feeling for the retrieval strings. These strings are made of thin, flexible plastic and are the only part of the device that should be accessible outside of the cervix. Checking for the strings, often done at the end of the menstrual cycle, confirms that the IUD is still resting correctly within the uterus.
The provider will teach the patient how to locate their cervix, which feels firm and rubbery, and how to feel for the thin strings hanging down from it. The strings should feel the same length each time they are checked; a noticeable change in length suggests the IUD may have shifted. The inability to feel the strings at all, or feeling the hard plastic of the IUD itself, are signs of possible displacement or expulsion.
Displacement means the IUD has partially slipped into the lower uterine segment or the cervix, which can compromise its effectiveness. If a change in position is suspected, a backup method of contraception should be used immediately, and a healthcare provider should be contacted for an evaluation. The provider can use a pelvic exam or an ultrasound to confirm the device’s precise location and determine if it needs to be repositioned or replaced.