An intrauterine device (IUD) is a device your doctor places inside your uterus to prevent pregnancy. It's T-shaped, made of soft, flexible plastic and is about the size of a U.S. quarter.

Threads on the end of the IUD extend from the opening of the uterus (cervix) into the upper vagina. Neither you nor your partner can feel them during intercourse. They're there to help you check that the IUD is in place and to help you remove it.

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Cervix The narrow neck of the uterus is called the cervix. << ENLARGE

Use of the IUD waned in the 1970s after studies linked one particular brand to serious health problems such as pelvic inflammatory disease (PID) — an infection of the female reproductive organs that can cause infertility and increase the risk of complicated pregnancy. However, today's IUDs are safe and even more reliable than the birth control pill. Understand how these devices work and what their advantages and risks are to help you determine whether they might be the right contraceptive for you.

IUDs prevent pregnancy by stopping sperm from meeting with an egg. Some also work by changing the lining of the uterus so that a fertilized egg can't implant and thickening cervical mucous so that sperm can't enter the uterus.

Your doctor usually can insert an IUD at any time during your menstrual cycle. However, he or she may want to place it during your period to ensure that you're not pregnant. If you've just had a baby, you must wait for six weeks after you deliver. This reduces the risk of the IUD coming out by itself (expulsion), which would leave you unprotected against pregnancy.

You may have cramps during the insertion and minor bleeding afterward. A few women may also experience continual or periodic light bleeding for three to six months afterwards. If you wish, you can exercise, swim, use tampons and have sex immediately after placement of your IUD.

Each month after your period, check that your IUD is in the right place by inserting a clean finger high into your vagina to locate the threads. If you can't feel them, or if you can feel the IUD itself, call your doctor. You need to have the IUD reinserted.

Your doctor can remove your IUD at any time. In most cases, the ability to become pregnant returns rapidly.

Both IUDs currently on the market are more than 99 percent effective at preventing pregnancy. This means that out of 100 women who use an IUD for a year, on average only one will become pregnant. Here's a profile of each type:

Mirena

• At-a-glance. Mirena (Mih-REH-nuh) releases a small amount of levonorgestrel (le-vo-nor-JES-trel) into the uterus. Levonorgestrel is a progestin — a synthetic form of the female sex hormone progesterone — that's often used in birth control pills.
• How long it lasts. Up to five years.
• Effectiveness rate. 99.9 percent.
• Advantages. Very light menstrual flow and less menstrual cramping than if you don't use an IUD.
• Disadvantages. Irregular light bleeding for several months is common.

ParaGard T 380A

• At-a-glance. Unlike Mirena, ParaGard doesn't contain hormones. It contains copper, which seems to kill sperm and add to its effectiveness.
• How long it lasts. Up to 10 years.
• Effectiveness rate. 99.2 percent.
• Advantages. Ability to become pregnant returns quickly after removal. Good option if you can't or don't want to use hormonal methods of birth control.
• Disadvantages. May cause cramping and longer, heavier periods.

As with any birth control method, IUDs aren't without side effects. Discuss these with your doctor. They include:

• Increased cramping, particularly during the first few months after insertion.
• Menstrual irregularities — either heavier or lighter periods.
• Expulsion of the IUD. Expulsion occurs in 5 percent of women in their first year of using an IUD. If this happens, you could become pregnant.
• Increased risk of pregnancy occurring outside the uterus (ectopic pregnancy), though this is rare.
• Piercing (perforation) of the uterus at the time of insertion.
• Pelvic infections may be more difficult to cure and more severe with an IUD in place.

Doctors used to think that IUDs raised the risk of pelvic inflammatory disease. This risk was largely related to a specific brand of IUD that's no longer available. Today's IUD's don't appear to increase the risk of pelvic infection. In fact, one study found that the greatest risk of PID was associated with IUD insertion and occurred in about 1 in 600 insertions.

No. If you're sexually active, an IUD won't protect you from human immunodeficiency virus (HIV) — the virus that causes AIDS — or AIDS itself. It also doesn't offer protection against any other sexually transmitted disease (STD). In fact, STDs can be more serious in women who use IUDs. That's why doctors advise that you only use an IUD if you're in a mutually committed relationship.

The exam, IUD, insertion of the IUD and follow-up visit may cost up to $450, according to the Planned Parenthood Federation of America.

An IUD is available by prescription only. At your initial visit with your doctor, he or she helps you determine if an IUD is an appropriate birth control option for you.