There are two methods used by IUDs sold in the United States. One way is by the encouragement of Mucuse production to create a barrior to the sperm. This is the method of the Mirena IUD which secretes progestin for this purpose. The Mirena IUD provides double protection against pregnancy because it also affects the lining of the uterus to prevent implantation of a fertilized egg. The copper ParaGard IUD does not have hormones. It works by activating enzymes in the lining of the uterus, which prevents the implantation of an egg.
Thought you would want to know about the implantation thing in terms of birth control.
Go back and read my original - I know how the IUD works and don’t advise it or dispense.