What it is:
An intrauterine device (IUD) is a T-shaped pliable plastic rod with an attached thread that measures just over an inch long and is inserted into the uterus through the cervix. There are two types on the market: One type is hormonal and lasts up to five years; the other uses copper and lasts up to 10 years.

What it does:
Although experts do not completely understand how the IUD prevents pregnancy, they believe the device works this way: It causes just enough tissue disturbances in the uterus to create an unfriendly environment for sperm. Few, if any, sperm can make it through the uterus to the fallopian tubes, so fertilization can’t occur.

There are two types of IUDs, and both also release substances that prevent pregnancy:

Progestin-releasing IUD (Mirena, made by Berlex):
Like other progesterone-based forms of contraception, this device causes the cervical mucus to thicken, which keeps sperm from reaching an egg. The hormone also thins the uterine lining, preventing an egg that does get fertilized from implanting. The Mirena lasts up to five years.

Copper-coil IUD (ParaGard, made by Barr Pharmaceuticals):
This IUD slowly releases copper ions for up to 10 years. Copper inhibits sperm and the egg’s ability to implant in the uterine wall.

Pros:
Although just 2 percent of American women use IUDs, of those who do, 99 percent report being satisfied. Once the device is inserted, nothing has to be done except a monthly self-check to make sure the string is still in place.

Which is better, ParaGard or Mirena?
It depends. If you crave a lighter, and possibly nonexistent, period, Mirena may be the way to go. Because it thins the uterine lining, there’s less blood, says Ross Marchetta, M.D., an ob-gyn in Akron, Ohio.

By contrast, the copper ParaGard can make bleeding and cramps worse by irritating the uterine lining, so ParaGard might not be for you if you already have severe cramps. However, if you want to avoid pumping artificial hormones into your body, ParaGard is a more appealing option.

Cons:
The main downside of IUDs is infection. While the risk is low, the devices can make existing ones—specifically, chlamydia and gonorrhea—much worse. About one in 100 women report infections in the first 20 days after getting an IUD, usually because bacteria have crept in during insertion. Those infections can generally be treated with antibiotics.

So finding a skilled doctor to perform the procedure is crucial. While all gynos are trained to do it, it’s a good idea to ask how often she inserts IUDs. Two or more a week is reassuring.

Also, women who’ve never been pregnant could possibly expel the IUD. Reason: Before pregnancy, your uterus and cervix are smaller and less pliable, so your body might resist having a piece of plastic resting there. But only about 2 percent of women have this problem.

The most common side effects associated with using the ParaGard IUD are cramping and heavy bleeding in some women. Women using the Mirena IUD may initially have irregular periods and bleeding. After a few months, you may experience lighter periods or no periods at all.

So why do IUDs have such a sketchy reputation? You can blame it on an old version called the Dalkon Shield, an IUD popular in the 1970s. It was pulled from the market in 1974 because it played a role in thousands of infections, such as pelvic inflammatory disease (PID) and sepsis. The conditions caused infertility in some women and have been linked to at least 17 deaths. But in 1974, the pocket calculator was considered high-tech—so comparing the old IUD with today’s versions is like pitting Pong against Halo.

Protects against STDs?
No

Prescription needed?
Yes