Does an IUD Always Prevent Pregnancy?

Content Courtesy of Harvard Health

Q: What are the chances of an IUD not working? Can you get pregnant with an IUD?

A: No method of birth control is perfect, but the intrauterine device (IUD) is one of the most effective ways to prevent pregnancy. The risk of getting pregnant when an IUD is in place is less than 1 percent.

Another advantage of the IUD, compared to other birth control options, is convenience. Once it is placed, the woman does not need to do anything for it to work.

There are two different IUDs available in the United States. Both are small plastic devices. One contains copper (ParaGard). The other contains a progesterone-like hormone (Mirena). The ParaGard device lasts up to 12 years. The Mirena type is changed after five years.

The progesterone-like hormone in the Mirena acts within the uterus and only minimal amounts actually get into the blood stream. Women who have significant menstrual cramps or heaving bleeding may prefer this device. Most women notice a decrease in cramping and less blood loss. But it can vary.

The IUD does not prevent sexually transmitted infections (STIs). It is not a good choice for women who are at increased risk for STIs. This includes women who have had a prior pelvic infection and those who are not in mutually monogamous relationships. The IUD should not be used in women who have an abnormally shaped uterine cavity.

When pregnancy does occur in a woman using an IUD, there is a chance that it is an ectopic pregnancy. An ectopic pregnancy is a potentially dangerous situation in which the pregnancy starts to develop outside of the uterus, usually within the fallopian tube. This is a rare complication, but it highlights how important it is for all IUD users to call their doctor or gynecologist if they experience lower abdominal or pelvic pain or a new vaginal discharge.

(Joan Marie Bengtson, M.D., is Assistant Professor of Obstetrics, Gynecology and Reproductive Biology at Harvard Medical School and a member of the Department of Obstetrics, Gynecology & Reproduction at Brigham and Women's Hospital, Boston, MA.)

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