Is it true that the copper iud allows fertilization but not implantation?

It blocks both.: Copper t iuds both prevent conception and implantation should conception occur. They are an excellent method for long-term reversible contraception that is cost-effective and grossly underutilized in the United States. Talk to your gynecologist to determine if that is a good option for you (best for stable, mutually monogamous relationships).

No: Both types of iud change the environment in the tube (where fertilization takes place) so fertilization doesn't occur.

Works against sperm!: Copper ions prevent pregnancy by inhibiting the movement of sperm, because the copper-ion-containing fluids are directly toxic to sperm.