This newly available hormone-releasing intrauterine device (IUD) is the single-most effective method of contraception in the world today--more effective, even, than sterilization--and it already has a 10-year record of safe, dependable use in Europe. The Mirena IUD is a welcome addition to the growing array of contraceptive choices in the U.S., but it's not for everybody.

Berlex Laboratories, Inc., U.S. subsidiary of the German drug firm Schering AG, received FDA approval in December of 2000 for a new intrauterine device (IUD) to be marketed as Mirena.

There are two other IUDs available in the U.S. at present: a copper-releasing device (Paragard, Ortho Pharmaceutical Corp) and one that releases the female hormone progesterone (Progestasert, Alza Corp). In contrast, Mirena relies on the release of levonorgestrel, a synthetic female hormone used in several different contraceptive products.

How it works

In place in the uterus, Mirena releases levonorgestrel at an initial rate of 20 micrograms (mcg) per day. Over the next five years, the rate gradually decreases to 10 mcg/day, but that's still more than adequate for contraceptive purposes.

How the local release of levonorgestrel prevents pregnancy has yet to be fully explained. Several effects, such as thickening cervical mucous and changes in the uterine lining, work together to impede the upward movement of sperm cells from the vagina to the prospective site of fertilization in a fallopian tube. Levonorgestrel also disrupts ovulation and probably interferes more directly with fertilization, but precisely how is not known.

Contrary to popular belief, IUDs do not induce abortion of an implanted fertilized egg--and that's true of all IUDs, including Mirena.

Highly effective

According to the authoritative medical reference Contraceptive Technology, the 1-year failure rate for Mirena is 0.1% (that's 1 in 1,000 women), lower than any other contraceptive method, even sterilization. The cumulative probability of pregnancy over seven years of continuous use is 1.1%.

What are the risks?

Mirena has been available in Europe for 10 years. Worldwide, about two million women have used it. It has compiled a good record of safe and effective service, but there are specific concerns about IUDs as a group.

Ectopic pregnancy -- Ectopic pregnancy is the development of a fertilized egg outside the womb. It is a very serious condition that threatens the life of the woman. Unfortunately, there is a fairly widespread misconception that IUDs increase the risk of ectopic pregnancy. IUDs do not increase the risk of ectopic pregnancy. It's just that IUDs are far better at preventing uterine pregnancy than preventing ectopic pregnancy, so a high percentage of the pregnancies that do occur with an IUD are ectopic. In clinical trials of Mirena, 50% of the very few pregnancies that did occur were ectopic, but the overall one-year rate of ectopic pregnancy was just 1 per 1000 women, the same rate found in sexually active women using no form of birth control.

Expulsion of the IUD -- Mirena can be left in place up to 5 years, but its contraceptive effect ends immediately upon removal. This is great for women using Mirena who decide they want to become pregnant; 80% of women attempting to become pregnant after using Mirena do so within one year of its removal. But a short contraceptive "half-life" also means if a woman expels the device, she will very shortly be at risk for pregnancy unless other contraceptive precautions are taken. Between 2% and 10% of women who try an IUD, any IUD, spontaneously expel the device within the first year, and this can occur without the woman's knowledge.

Women using Mirena need to regularly verify the device is still in place by feeling inside the vagina for two fine monofilament strings that hang down from the bottom of the IUD frame. The strings should always be palpable, and their length should always feel the same--any perceptible shortening of the strings suggests the device is out of correct position and will require adjustment by a medical professional.

No protection against HIV or other sexually transmitted infections (STIs) -- IUDs provide no protection against STIs. As such they are generally recommended only for women in stable, mutually monogamous relationships (and this assumes both partners are free of STIs).

Pelvic inflammatory disease (PID) -- PID is usually caused by infections (including STIs) that move upward into uterus, fallopian tubes, or ovaries. PID increases the risk of ectopic pregnancy, can result in infertility, and, very rarely, even in death. A number of papers over the years have suggested that IUDs play a role in promoting the spread of infections into the upper genital tract by providing a pathway for microbes. Any woman who has an IUD inserted is considered to be at increased risk of PID for the first 20 days. Also, IUD users who acquire an STI may be at greater risk for PID than other women.

Despite these risks, it's important to note that IUDs, in and of themselves, do not increase the long-term risk of PID. The critical factor is the presence of STIs or the possibility of microbes being present at the time of insertion. Close monitoring for PID symptoms in the initial 20 days and staying clear of STIs have been cited as important safeguards for IUD users. The early risk of PID with Mirena is lower than with the copper-releasing IUD.

Perforation -- With any IUD, there is a small risk of perforation of the cervix or uterus. In the event of perforation the IUD must be removed, and surgical repair may be necessary.

What about side effects?

Because Mirena releases hormones directly to the targeted tissues as opposed to delivering them via the circulatory system, the potential for hormonal side effects is reduced, compared with oral, injectable, and implantable hormones. The more important potential side effects associated with Mirena include the following:

Irregular bleeding and amenorrhea -- In the first three to six months of Mirena use, the number of days a woman has intermittent menstrual bleeding and spotting may increase, and her overall menstrual pattern may become irregular. The number of bleeding and spotting days should decrease after six months, but her menstrual pattern may continue to be affected. After one year's use, about 20% of women using Mirena stop having periods altogether (amenorrhea). Some women regard this as a benefit. However, amenorrhea may be a nuisance initially because a missed period could also mean pregnancy, which requires special medical attention for Mirena users. (Mirena should be removed if the user becomes pregnant.)

Ovarian cysts -- Ovarian cysts, which are fluid-filled sacs or pouches that develop on the surface of the ovaries, are found in about 12% of women using Mirena. They can cause pelvic pain or pain during intercourse, but in most cases they resolve on their own within two to three months, and surgery is not required. Ovarian cysts are quite common in all women of child-bearing age, regardless of contraceptive method(s) used.

Other adverse reactions -- Cramping, dizziness, or faintness while Mirena is being inserted are common. Cramping may be severe. In clinical studies, adverse events noted in women using Mirena ranged from abdominal pain and headache to skin problems. Many of these events were of short duration, and many were probably unrelated to Mirena.

User satisfaction, an indicator of side-effects tolerability, is very good with Mirena; 81% of women who try it stay on it at the end of one year, compared with 71% for birth control pills, 61% for the male condom, and 56% for the diaphragm. Only the Norplant implantable contraceptive has a higher one-year continuation rate, at 88%. (It should be noted that the removal of a contraceptive implant is a surgical procedure, significantly more costly and involved than removing an IUD.)

Is it right for me?

Again according to Contraceptive Technology, worldwide about 12% of married women rely on an IUD for contraception, but in the U.S., less than 1% of women "at risk for pregnancy" use an IUD. In the 1970s, that number was as high as 10%, but the highly publicized recall of the Dalkon shield, concerns about infertility, and the potential legal consequences for prescribing physicians decimated the U.S. market.

Today's IUDs are nothing like the Dalkon shield, and many of the general concerns about their safety and mode of action have been exposed as myths. The biggest concerns are probably the increased risk of PID in the 20 days immediately following IUD insertion and the fact that IUD users who acquire an STI are at increased risk for PID.

Mirena is recommended for women who have had at least one child; are in a stable, mutually monogamous relationship; have no history of PID; and no history of ectopic pregnancy or condition that might predispose them to ectopic pregnancy. Those who fit this profile now have access to a contraceptive product with an outstanding safety and effectiveness record among European women.