Hormone-releasing IUDs constantly release small amounts of steroid hormone into the uterus. The Progestasert, which has been marketed since 1976, contains 38 mg of progesterone released at a rate of 65 micrograms per day for one year. The LNG-20 IUD contains 52 mg of levonorgestrel released at a rate of 20 micrograms per day and lasts at least five years (610). Since the early 1990s it has been approved for use in Denmark, Finland, Norway, and Sweden (497, 610). Recently, the LNG-20 has also been approved in Belgium, France, Iceland, Singapore, Switzerland, and the United Kingdom.

The Lippes Loop, made of polyethylene, was the most widely used unmedicated IUD outside China. This IUD is no longer distributed internationally. The other major types of unmedicated IUDs are flexible stainless steel rings (either round or in the shape of the uterine cavity and made with a single or double coil). These rings have been widely used in China but not elsewhere.

Researchers continue to develop and test new IUDs that may reduce expulsion rates and side effects. Among the devices being considered are a smaller, lightweight, and flexible T-shaped copper IUD, the Cu-SAFE 300, which can be inserted without a plunger and is designed to move towards the uterine fundus (the top of the uterus) when the uterus contracts (522, 643). A frameless IUD consisting of six copper sleeves on a surgical nylon thread is called the FlexiGard 330, GyneFix™, or CuFix PP330. The thread is knotted at one end, which is anchored in the muscle of the fundus. In clinical trials the device has proved to be highly effective and comfortable to use (492, 563, 625, 641, 642, 647, 649). Researchers in Switzerland have taken yet another approach: a copper-bearing IUD with a T-shaped frame, called the Sof-T. The tip of each end of the arm consists of a soft ball, designed to prevent perforation and to block the openings to the fallopian tubes in order to prevent sperm from entering (500, 581).