Like the famous battery, the IUD "keeps working, and working, and working." Depending on the brand, it will protect you from pregnancy for up to 10 years. And, best of all, you don't have to do anything: There's no pill to remember or device to insert before sex. Some factors, however, including your age, your childbirth history, and your doctor's experience with IUD insertion, can reduce its effectiveness.

Among women who use the IUD carefully (checking strings regularly to detect expulsion), the probability of pregnancy during the first year of use is 0.6 percent for the Copper-T, 1.5 percent for the Progestasert, and 0.1 percent for the Mirena. For typical users (those who are a bit less careful), the failure rate is only slightly higher—0.8 percent for the Copper-T, 2.0 percent for the Progestasert, and 0.1 percent for the Mirena.