How does a Copper IUD Work?

In vivo, copper IUDs exert a pre-fertilization effect.

Copper ions reduce the number and function of sperm and oocytes

Black D. and Waddington A. et al. Build Your Expertise: IUC slide deck: 36,62.
SOGC Clinical Practice Guideline, Canadian Contraception Consensus (part 3): Chapter 7 – Intrauterine Contraception