How does an IUD work? How effective is it?
You've raised an interesting question and the answer is somewhat complex. An IUD (intrauterine device) is a small t-shaped device that your healthcare provider inserts into the uterus to prevent pregnancy. There are two types of IUDs commonly used in the US, they're effective for 5 to 10 years depending on which one you choose.
As for how these devices work, researchers really aren't sure. There are a couple of prevailing theories, but again, the exact mechanism behind how an IUD prevents pregnancy is debatable. One school of thought, is that an IUD acts as a "foreign body" in the uterus. To explain, the lining of the uterus, the endometrium, is very sensitive to both hormonal stimuli and also to things occupying the uterus. The uterus itself usually has an increased tone (muscular contraction) which is part of its tendency to want to expel anything distending it, be it a baby, a period's blood clot, or an IUD. This is why many women with IUDs can have rougher periods than before they had one inserted—the body tries a little harder to get the device out. The foreign body idea goes back to antiquity, including stories of rocks or fruit pits being inserted into the uteri of livestock to keep them from becoming pregnant.
Recently, more studies have suggested that the IUD does other things to prevent conception, namely, adversely affecting sperm transport before conception and altering the motility of the tubes, necessary for an egg (fertilized or not) to travel down toward the uterus. (Fertilization takes place in the fallopian tube before entering the uterus.)
Also, IUDs today are made with copper, which is spermicidal, or with progesterone, which makes the cervical mucus more hostile and impenetrably thicker while making actual ovulation less likely.
Now here comes the complexity—as I mentioned researchers are not exactly sure how an IUD works. It could be that the device prevents the sperm from properly entering the fallopian tube to fertilize the egg or it could be that the fertilized egg can't properly implant in the uterus because of the presence of the IUD. So depending on your views of when life begins and whether terminating a pregnancy is appropriate you might want to think about whether or not to use an IUD. I make this point purely as a point to consider, it's completely up to you and your partner. But it could very well be that the IUD is causing your body to expel a fertilized egg, not just keeping sperm from reaching the egg.
As far as the safety of an IUD, there's very little risk involved. Aside from the possibility of the more painful periods mentioned above, the only other risk is that it will perforate (pop through) the wall of the uterus either at insertion or on its own. If it's copper, this metal is very irritating and can cause peritonitis-like symptoms. If it's progesterone loaded, even perforation won't necessarily cause problems.
Removal involves a doctor's merely pulling on the little residual string that can be seen at the cervix (mouth of the womb) during an exam. A momentary cramping will go away soon after removal. If the IUD is pivoted around during its tenure, the string may no longer be visible, and the IUD needs to be "fished" out, sometimes requiring anesthesia.