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It is not known fully how IUDs work. They are thought to prevent conception by causing a brief localized inflammation that begins about 24 hours after insertion.
This causes an inflammatory reaction inside the uterus that attracts white blood cells. The white blood cells produce substances that are toxic or poisonous to sperm. The progesterone-releasing IUDs cause a subtle change in the endometrial environment that impairs the implantation of the egg in the uterine wall. This type of IUD also alters the cervical mucus, which, in turn, inhibits sperm from passing through the cervix.
IUDs are only available by prescr*iption and must be properly inserted by a health care professional. A pelvic exam is required to insert an IUD. The IUD is usually inserted into the uterus during a woman's menstrual period although it can be done at any time during her monthly cycle as long as she is not pregnant.
The woman must check her IUD every month to be sure that the IUD is still in place. Sometimes, the uterus expels (pushes out) the IUD. The spontaneous expulsion rate is 10% during the first year of use, and it is even higher in younger women or in women who have never given birth. It is estimated that 20% of IUD expulsions are overlooked. In addition to the woman checking the IUD, the device must also be checked periodically by a health care professional.
More than 10,000 lawsuits were filed by women who had used the Dalkon Shield. These women claimed primarily that the Dalkon Shield, which had a multifilament string that allowed bacteria to travel from the vagina up into the uterus, caused them to contract pelvic inflammatory disease. The IUDs in use today have a monofilament nylon string (much like fishing line) that reduces the risk of pelvic inflammatory disease. As a result of this and other improvements, both the World Health Organization (WHO) and the American Medical Association (AMA) consider IUDs to be one of the safest and most effective forms of reversible contraception that a woman can use.
http://www.medicinenet.com/iud/page2.htm