In the third and final installment of our series, we examine the safety profile of the intrauterine device (IUD) and consider the ideal user.

Let's take a moment to review some of the major advantages of extended-use IUDs:

Effectiveness -- The Mirena progestin-releasing IUD (5-year service life) and the ParaGard copper IUD (10-year service life) are the most effective forms of reversible birth control in the world, with low failure rates rivaling those of male and female sterilization.

Economy -- Despite the relatively high cost of the device, including insertion and removal by a trained healthcare professional, an extended-use IUD is the most cost-effective form of reversible contraception if left in place for at least two years. The approximate annual cost of the ParaGard copper IUD, for example, "amortized" over its 10-year service life, ranges from around $20 to $50 a year, depending on the healthcare setting in which it is supplied (HMO, private practice, public clinic, etc.)

Ease of use -- Other than the need for periodic checking to confirm the device is still in place, an IUD requires no decision or action on the part of the woman, and it is always ready on demand.

But is it safe?

Ultimately, the best test of the safety of a drug or medical device is its long-term use in the general population. Copper IUDs like ParaGard have been used safely by millions of women around the world for more than 20 years. The Mirena IUD has been available in Europe for more than 10 years and has been used worldwide by about two million women. Properly inserted, in a good candidate with no prohibitive risk factors, IUDs are among the safest methods of birth control. In absolute terms, when considering the risk of death associated with the various contraceptive methods plus the risk of death associated with pregnancy should the method fail, the only option safer for sexually active women than the IUD is male sterilization (vasectomy). However, there are other safety concerns--some real, some fallacious--that need to be addressed:

No protection against sexually transmitted infections (STIs) -- IUDs are designed to do one thing: prevent pregnancy. They offer the user no protection against STIs. This is why IUDs are recommended only for women in stable, mutually monogamous relationships in which both partners are free of STIs.

Increased risk of pelvic inflammatory disease (PID) -- PID is usually caused by an STI that moves from the vagina into the upper reproductive tract (uterus, fallopian tubes, ovaries). PID increases the risk of ectopic pregnancy and can result in infertility. One way the upward movement of STI is facilitated is the insertion of an IUD, and any woman who has an IUD inserted is at increased risk of PID for the first 20 days. Also, a woman with an IUD in place who acquires an STI is at greater risk for PID than if she were using another form of contraception. Close monitoring for PID symptoms in the initial 20 days of use and subsequent avoidance of STIs are usually adequate safeguards.

Ectopic pregnancy -- When a fertilized egg begins to develop anywhere outside the uterus, the pregnancy is termed ectopic. Ectopic pregnancy is a serious condition that can cause death. Contrary to popular belief, IUDs do not increase the risk of ectopic pregnancy. IUDs are just better at preventing normal pregnancy than ectopic pregnancy, so a greater proportion of the few pregnancies that do occur with an IUD are ectopic. In studies of Mirena, half of the few pregnancies that did occur were ectopic, but the overall one-year rate of ectopic pregnancy was just 0.1% (1 per 1000 women), which is about the same as the ectopic pregnancy rate for sexually active women using no birth control.

Expulsion of the IUD -- Between 2% and 10% of women who have an IUD inserted will spontaneously expel the device within the first year, and 20% of expulsions go unnoticed. Expulsion is more likely among younger women and women who have never had a baby. Strenuous physical activity does not increase the risk of expulsion. The contraceptive protection of an IUD ends immediately upon its removal, so if a woman expels the device, she will very shortly be at risk for pregnancy unless other contraceptive precautions are taken. A third of pregnancies occurring among IUD users are actually the result of the device being expelled. A woman using an IUD should regularly check that the device is still in place by feeling inside her vagina for the monofilament string(s) hanging down from the bottom of the T-shaped frame.

Perforation -- With any IUD, there is a small risk of perforation of the uterus (0.1% to 0.3%). This sounds painful but usually isn't, and most cases are noticed right away and corrected. Sometimes the IUD "migrates" through the perforation into other regions of the pelvis, in which case surgery may be required to remove the device.

Side effects -- Copper IUDs contain no hormones, and progestin-releasing IUDs deliver their hormones directly to the uterus, as opposed to systemically (via the bloodstream), so both types of IUD are devoid of the hormonal side effects associated with the Pill or hormonal implants. Most of the side effects associated with everyday use of an IUD are menstrual irregularities. Spotting between periods is common with all IUDs. The ParaGard copper IUD may initially cause an increase in menstrual flow, and in some women, this increased bleeding may cause anemia. The progestin-releasing Mirena IUD, on the other hand, tends to decrease menstrual flow (sometimes stopping it altogether). Paragard may increase menstrual cramping while Mirena may lessen it.

One way of gauging the severity of contraceptive side effects is to look at how many women continue using a method after one year. The one-year continuation rate for the Mirena progestin-releasing IUD is 81%; for the ParaGard copper IUD, 78%. Compare those rates with the continuation rates for the Pill (71%), the diaphragm (56%), and spermicide (40%).

Is there an ideal IUD candidate?

IUDs are not for everyone. Because of the risk of STI and subsequent PID, only women in stable, monogamous relationships should consider using an IUD. There are other restrictions based on a woman's sexual and reproductive health that must be considered as well. No woman should be fitted with an IUD until she has had a thorough pelvic examination, been tested for a number of STIs, and been tested for anemia. If you are considering an IUD, remember that many practitioners have little or no experience with IUDs because so few women in this country use them. Training is critical to ensure proper insertion and removal, and it may be helpful to ask your healthcare provider how much experience he or she has had with IUDs.

Expulsion of the IUD is more common among women who have not given birth (nulliparous women), and manufacturers advise limiting IUDs to women who have had at least one child. But according to Paul Feldblum, MD, senior epidemiologist at Family Health International, nulliparity does not have to preclude IUD use. "The key factor, " says Feldblum, "is exposure to STIs, because those infections can lead to later infertility. But any woman who is confident she will not contract an infection can use an IUD, no matter how many children she's had."

"The need for safe, effective, affordable, long-term, compliance-free contraception is obvious," Feldblum adds. "The IUD can fill this role for millions of women, in the U.S. and around the world."