An IUD is a small, T-shaped plastic device roughly the size of a quarter. It sits inside the uterus with its two short arms spread horizontally and a thin vertical stem pointing downward toward the cervix. Thin strings hang from the bottom of the stem and trail through the cervix into the upper vagina, where they can be felt with a fingertip.
The Basic T-Shape
Every IUD on the market shares the same general silhouette: a capital letter T. The design dates back to 1967, when researcher Howard Tatum found that a T-shaped frame conformed to the triangular uterine cavity better than earlier straight or coiled designs, leading to fewer expulsions. The frame is made of flexible polyethylene, a lightweight medical-grade plastic. Barium sulfate is mixed into the plastic so the device shows up clearly on X-rays if imaging is ever needed.
The two horizontal “arms” of the T sit near the top of the uterus, and they’re flexible enough to fold down during insertion and spring back open once inside. The vertical stem runs down the center. Attached to the very bottom of the stem are two thin monofilament strings that extend about one to two inches past the cervix. Although there are two strings, they typically feel like a single thread when you check for them.
How Copper and Hormonal IUDs Look Different
The easiest way to tell a copper IUD apart from a hormonal one is color. The copper IUD (Paragard) has thin copper wire visibly coiled around its vertical stem and along a section of each arm. This gives it a distinctive metallic, reddish-gold appearance against the white plastic frame. It’s the only non-hormonal IUD available in the U.S.
Hormonal IUDs are all-plastic in appearance, typically white or off-white. Instead of copper wire, they have a small silicone reservoir built around the central stem that slowly releases a hormone to prevent pregnancy. The arms on hormonal models are slightly bowed rather than perfectly straight, giving them a softer curve compared to Paragard’s rigid, angular arms.
Size Differences Between Brands
IUDs are small, but they aren’t all the same size. The differences come down to a few millimeters, which can matter for comfort, especially in people who haven’t been pregnant before.
- Mirena and Liletta are the largest at 32 mm by 32 mm (about 1.25 inches in each direction). These are the full-size hormonal options.
- Kyleena and Skyla are slightly smaller at 28 mm by 30 mm, designed with a narrower frame that may be easier to place in a smaller uterus.
- Paragard (the copper IUD) has a similar overall footprint to Mirena and Liletta but looks bulkier because of the wrapped copper wire.
To put those numbers in perspective, even the largest IUD is only slightly wider than a U.S. quarter (which measures about 24 mm across). The entire device fits comfortably in the palm of your hand.
One Small Visual Detail on Skyla
Skyla has a unique feature not found on other hormonal IUDs: a tiny silver ring embedded on the stem at the base of the arms. This ring serves as a marker that makes Skyla easier to identify on ultrasound. It’s not visible once the device is placed, but if you ever see an image of the IUD brands side by side, that small metallic band is how you can spot a Skyla.
What an IUD Looks Like on Imaging
After insertion, your provider may use an ultrasound to confirm the IUD is sitting in the right position. On ultrasound, the device appears as a bright white T-shape against the darker uterine tissue. The copper Paragard tends to be the most vivid on the screen because the copper wire reflects sound waves strongly, creating a bright, echogenic image. Hormonal IUDs are slightly less dramatic but still clearly visible, especially with transvaginal ultrasound, which uses a higher-frequency probe for sharper resolution.
Three-dimensional ultrasound is particularly useful because it can display the coronal view of the uterus, showing both arms of the T in relation to the uterine walls in a single image. This makes it easy to confirm the IUD is centered and not tilted or partially expelled. If the device can’t be found on ultrasound, a standard pelvic X-ray or CT scan can locate it, since the barium in the plastic frame makes every IUD show up on radiographic imaging regardless of brand.
The Strings You Can Feel
The part of an IUD you’re most likely to encounter yourself is the strings. They’re made of thin, flexible monofilament (similar in texture to fishing line, though much thinner) and hang just past the opening of the cervix. After insertion, your provider trims them to a length that keeps them tucked inside the vaginal canal and out of the way.
Over time, the strings soften and curl against the cervix, making them less noticeable. When you reach in to check them, you’re feeling for a short, thread-like tip at the top of the vaginal canal. You won’t feel the T-shaped body of the device itself, since that sits higher up inside the uterus, well beyond the cervix. If you can suddenly feel hard plastic rather than just a string, or if the strings seem significantly longer or shorter than usual, that’s a sign the IUD may have shifted from its original position.