First, it is important to realize that IUDs are a very safe and effective form of birth control. However, like everything in medicine, there are some risks. In deciding whether or not an IUD is the right form of birth control for you, it is important to understand these risks and compare them with the risks of other forms of birth control.
Two of the most concerning risks are for infection and uterine perforation. For the first three weeks after an IUD is inserted, a woman is at increased risk for developing pelvic inflammatory diseases (PID). This is where there is an infection of the uterus and fallopian tubes that can extend to the ovaries and abdomen. It can result in pain, fever and scarring of the fallopian tubes that can result in infertility. The infection can also progress to cause sepsis and even death. However, the IUD doesn't cause an infection. If there is an infection of the cervix at the time when the IUD is inserted (or shortly thereafter) there is an increased risk that this infection can extend from the cervix into the uterus and fallopian tubes. So, if you don't have an infection and are not exposed to the bacteria that causes the infection for the first 21 days after the IUD is inserted, you are not at increased risk for developing PID.
The other concerning risk is for uterine perforation. This generally occurs at the time of IUD insertion and involves the IUD entering into the wall of the uterus and sometimes going all the way through the uterus and into the abdomen. Once inside the abdomen, the IUD will usually come to rest behind the uterus but it could also travel to other sites in the abdomen. In the case of an IUD perforation, a surgical procedure is required to retrieve the IUD. Thankfully, this is a rare complication and only occurs in about 1:1000 insertions.
The two forms of IUD available in the United States (copper and hormonal) each have their own risks as well. Often, choosing which form of IUD is best for you involves choosing between these risks. The copper IUD can make your period (menses) heavier. For women with very light periods, this is usually not a problem, but for women with very heavy periods, this can cause low blood counts (anemia) and make it more difficult to maintain a healthy blood count. However, bleeding or spotting between periods is rare with the copper IUD. In contrast, the hormonal IUD usually makes periods lighter- in fact some women stop getting their periods altogether. However, it is more common to get spotting between periods when using the hormonal IUD. So, for a woman who would rather have a slightly heavier period but not have spotting between periods, the copper IUD might be better. But, for a woman who would rather have a lighter period and can tolerate some spotting between periods, the hormonal IUD might be ideal.
While IUDs are very effective forms of birth control, they are not perfect and an unplanned pregnancy can occur even with a properly inserted IUD. If a pregnancy does occur, it is possible to be an ectopic pregnancy. While it is a confusing point, it is important to understand that the overall risk of an ectopic pregnancy is lower if you have an IUD then if you don't have an IUD. But, if you do get pregnant, the chance that is it ectopic pregnancy is higher in women with and IUD. Therefore, if you have an IUD and think you might be pregnant, check with your doctor right away.
IUDs also can be expelled from the uterus. This simply means that they fall out of the uterus and into the vagina (and usually just fall out of the body completely). This isn't dangerous, per se, but if the IUD is not inside your uterus, it cannot function properly as a birth control device and you would be at risk for pregnancy.
IUD's also do not protect against sexually transmitted infections (STI's). So, even though the IUD is very effective at preventing pregnancies, if you are engaged in a sexual activity that puts you at risk for acquiring an STI, you should use a condom as well.
There are a number of additional, but very rare, complications and contraindications, so you should discuss your particular situation with your doctor.
For more information, be sure to check out:
Types of Emergency Contraception
What You Can Expect After Taking the Morning After Pill
Emergency Contraception: How Much Do You Know About the Morning After Pill?
The Morning After Pill
Understanding the Risks of an IUD