Copper: As mentioned, IUDs seem to make it difficult for sperm to survive passing through the uterus to reach an egg. In addition to that, with copper-releasing IUDs, the copper appears to be toxic to sperm, acting as a spermicide.a Moreover, copper-containing IUDs are said to alter the lining of the uterus.

Hence, no one can say with certainty that IUDs that include copper or a hormone never allow for the fertilization of an egg. However, scientific evidence suggests that because of a number of mechanisms mentioned above, pregnancy rarely occurs with the use of such IUDs.