An intrauterine device is a small, plastic object shaped like a “T” that is placed in a woman’s uterus to prevent pregnancy. Also known as an IUD, this device is among the most effective and popular forms of birth control in the world. The IUD is about the size of a quarter and cannot be felt by the woman or her partner during intercourse.

IUDs come in two forms:

• Copper-containing IUD. In this device, the arms of the IUD are covered with a copper wire. The copper prevents fertilization by keeping sperm from traveling up the uterus into the fallopian tubes, where the egg is located.
  
  
• Hormone-releasing IUD. This device contains hormones. It prevents pregnancy by stopping the sperm and the egg from meeting on their way to the fallopian tubes. A levonorgestrel (a progestin) releasing version is available.

In the unlikely event that fertilization does take place, both forms of IUD offer further protection against an unwanted pregnancy by preventing the fertilized egg from implanting in the uterine wall.

Once intrauterine devices are inserted, they provide contraception for anywhere from five to 10 years, depending on the type used, or until they are removed. Less than 1 percent of women who use an IUD will become pregnant. However, these devices do not protect against sexually transmitted diseases (STDs), such as HIV infection, and in rare cases they can produce health risks.

Copper-containing IUDs can also be used as a form of emergency contraception. When inserted up to seven days after unprotected sexual intercourse, the IUD helps keep a fertilized egg from being implanted in the uterus, thus preventing pregnancy.