There are two types of iud (intrauterine device) otherwise reffered to as the coil, to prevent pregnancy. Both are over 99% effective. The copper coil doesn't use hormones. It prevents pregnancy by releasing tiny amounts of copper into the uterus. The copper affects the way sperm swim so they can't fertilise an egg. The Mirena is referred to as an IUS (intrauterine system) just to make things more confusing! It uses hormones that thin the lining of the uterus and prevent ovulation. Both of these