In part one of this three-part series, we examined reasons why the popularity of the intrauterine device (IUD) in the U.S. lags so far behind the rest of the world. Here, in part two, we'll have a look at the three IUDs currently marketed in this country.

Let's begin by talking about design similarities: all three IUDs available in the U.S. are basically small T-shaped devices with one or two monofilament tail-strings attached at the bottom of the vertical stem of the 'T'. The horizontal arms of the T-shaped plastic frame are collapsible for insertion and removal, but they remain extended when the device is in place in the uterus to keep the IUD from slipping out. The tail-strings serve two purposes: they allow the user to manually confirm that the IUD is still in place, and they facilitate its removal.

The differences between today's IUDs center on distinct methods of action and the duration of their effectiveness. The options:

MIRENA (Berlex Laboratories)

Mirena (muh-RAY-nah), approved in December of 2000, is the newest IUD on the U.S. market, but it has been available in Europe for 10 years. Worldwide, more than 2 million women have used this IUD.

Service life: Five years

Mechanics: The T-shaped plastic frame incorporates a reservoir of levonorgestrel, a synthetic female hormone that mimics progesterone, which is released at an initial rate of 20 mcg/day, decreasing gradually to 10 mcg/day over five years.

Action: The contraceptive action of levonorgestrel is not fully understood, but the following effects have been noted: thickening of cervical mucous; preventing passage of sperm into the uterus; inhibition of sperm viability; changes to the uterine lining; and inhibition of ovulation in some women.

Effectiveness: In the first year of use, only 1 woman in a 1000 (0.1%) will become pregnant using Mirena, which makes it even more effective than female sterilization (0.5%). The cumulative probability of pregnancy after 7 years of continuous use is only 1.1%.

PARAGARD (Ortho-McNeil)

The ParaGard "copper T" accounts for almost all IUD use in this country. Worldwide, more than 25 million of these devices have been distributed in 70 countries.

Service life: Ten years

Mechanics: The arms of the T-shaped plastic frame are surrounded by copper sleeves, and the vertical stem is wound with copper wire. The device releases copper directly into the uterus. There are no other active ingredients.

Action: The contraceptive effects of copper are not fully understood, but it has been hypothesized that it interferes with sperm transport, sperm viability, fertilization, and implantation.

Effectiveness: In the first year of use, only 8 women in a 1000 (0.8%) will become pregnant using ParaGard. The cumulative risk of pregnancy over 10 years of use is just 2.6%.

PROGESTASERT (ALZA Corporation)

Service life: One year

Mechanics: The T-shaped plastic frame incorporates a reservoir of progesterone, which is released at a rate of 65 mcg/day for one year.

Action: The contraceptive action of progesterone has not been fully characterized. Two mechanisms have been suggested: interference with sperm viability and alteration of the intrauterine environment to prevent implantation.

Effectiveness: In the first year of use, 2 women out of 100 (2%) will become pregnant using the Progestasert IUD.

How to choose?

Since IUDs offer absolutely no protection against sexually transmitted infection, it's probably safe to assume that effectiveness and side effects will be priorities to the woman shopping for an IUD. According to Contraceptive Technology, "Over the long run, the [Mirena IUD] is the single most effective method of reversible contraception available in the world today, followed closely by the [ParaGard copper IUD]."

With the recent approval of Mirena, it's difficult to say who might be a prime candidate for the Progestasert IUD, given it's comparatively high failure rate and the fact that it has to be replaced every year. Cost and availability through various insurance plans are the other factors to consider.

As for side effects, we will be discussing those and a number of other safety issues in the third and final installment of this series. For now, suffice it to say that both Mirena and ParaGard have excellent safety profiles, with the most pronounced side effects being menstrual changes. These side effects, however, can be an important consideration in weighing today's options.

"A woman's menstrual bleeding pattern, and her tolerance for heavier or lighter bleeding can help steer her to one IUD or another," says Paul Feldblum, PhD, senior epidemiologist in clinical research at Family Health International. "The copper IUD [ParaGard] tends to cause heavier and longer periods, although this effect lessens over time. Conversely, the newly approved Mirena hormonal IUD reduces bleeding and often results in amenorrhea (no periods). However, it's difficult to predict how severe these changes will be in any individual woman."

In the third and final installment of this series, we'll be looking at safety issues, including who should and shouldn't use an IUD.