Intrauterine devices (IUDs) are small, usually T-shaped devices inserted into the uterus by a healthcare professional. IUDs are a safe and effective (more than 99% actual effectiveness) form of contraception, but they are also a little mysterious. Scientists aren’t entirely sure how IUDs work. It is believed that they prevent sperm from reaching the egg, but they may also thin the uterine lining and prevent ovulation.

Once in place, an IUD can remain effective up to 10 years, depending on the type of device. Whenever the woman decides to become pregnant or to use another form of birth control, she must see her healthcare provider to have the IUD removed.