An IUD is inserted into the uterus by a health-care professional.
How IUDs prevent pregnancy is not entirely clear. They seem to prevent sperm and eggs from meeting by either immobilizing the sperm on their way to the fallopian tubes or by changing the uterine lining so the fertilized egg cannot implant in it.
IUDs have one of the lowest failure rates of any contraceptive method. " In the population for which the IUD is appropriate -- for those in a mutually monogamous, stable relationship who are not at a high risk of infection -- the IUD is considered a safe and effective contraception method.
However, the IUD's image suffered when the Dalkon Shield IUD, which was associated with a high incidence of pelvic infections, infertility and some deaths, was taken in 1975 off the market. Today, serious complication from IUDs are rare, although IUD users may be at increased risk of developing pelvic inflammatory disease. Other side effects can include perforation of the uterus, abnormal bleeding, and cramps. Complications occur most often during and immediately after insertion.
This is in part based on information from the U.S. Food and Drug Administration (the FDA).