They seem to do it by affecting the way the sperm or eggs move. They can also affect the lining of the uterus in ways that prevent implantation. Of 100 women who use IUDs, two or fewer will become pregnant during the first year of typical use. Fewer than one will become pregnant with perfect or typical use of the ParaGard. Two or fewer will become pregnant with perfect or typical use of the Progestasert.
Fewer pregnancies occur with continued use.
You can increase your protection by checking for the IUD string regularly. The IUD provides no protection against sexually transmitted infections. With an IUD in place, a woman does not need to think about using her birth control method every day or every time she has vaginal intercourse. The IUD does not change the hormone levels throughout the body. Although the up-front cost of the copper IUD is high, the cost over the ten years it can be used is very low compared to the costs of similarly effective methods.