An intrauterine device (IUD) is a type of contraception that is used all over the world. In the United States, there are two different versions.

The more traditional IUD is made of copper, while a newer iteration of the IUD is made of plastic and has a small compartment in it that is filled with a derivative of the hormone progesterone. This type of IUD is sometimes referred to as an intrauterine system (IUS). An IUD/IUS is T-shaped and about the size of a quarter; it is placed inside the uterus by a doctor or nurse. (The average uterus is the size of a clenched fist.) They are recognized as a form of “long-acting reversible contraception” because they can be left in the uterus and prevent pregnancy for 5-10 years. An IUD/IUS is one of the most cost efficient and effective types of contraception available [1]. They are the most common form of birth control in the world (45% of married women in China use it), although they are used by only about 1% of women United States. Americans tend to use birth control pills more than IUDs/IUSs.

The IUD and IUS are similar in size and shape and both devices cause a low-grade local inflammatory response that makes the uterine environment less hospitable to sperm or a fertilized egg. The IUD and IUS have additional mechanisms for preventing pregnancy that are unique. Copper ions released from the IUD enhance the inflammatory response and the presence of the copper ions is toxic to sperm. It is like a spermicide. In addition, the mucous in the uterus can become thicker, making it more difficult for sperm to make their way up to an egg. Finally, the copper-laden fluids of the uterus make the environment hostile to an embryo should an egg become fertilized.

The IUS, on the other hand, functions more like conventional contraceptive methods because it constantly releases levonorgestrel, a progesterone derivative. The constant level of progesterone prevents the uterine wall from becoming full of nutrients and thereby inhibits implantation. As with birth control pills, an IUS maintains a non-pregnant hormonal state so that the body cannot foster an embryo. Over time, there is some glandular atrophy and ovulation is prevented. Once it is removed, however, most women have no trouble getting pregnant if they want to.

[1] Mavranezouli I, Wilkinson C, Glasier A. The cost-effectiveness of long-acting reversible contraceptive methods in the UK: analysis based on a decision-analytic model developed for a National Institute for Health and Clinical Excellence (NICE) clinical practice guideline. Hum Reprod. 2008;23(6):1338-1345.