IUDs work by preventing an egg from being fertilized. (Substances released by the IUD immobilize sperm.) There are two kinds, one of which releases the hormone progestin, which thickens the cervical mucus. This acts as a barrier to prevent sperm from entering the uterus. I think it also affects the lining of the uterus by preventing an egg from being implanted. The other IUD type releases chemicals that affect the lining of the uterus by not allowing an egg to implant. It also affects the hormones needed to support a pregnancy. Supposedly the IUD is one of the most effective methods of contraception available although there is no protection against sexually transmitted infections.

IUD's work primarily by preventing sperm from fertilizing an egg. First, they stimulate an inflammatory response in the uterus, causing changes that damage or kill sperm and may damage an egg also. It is unlikely for an egg to get fertilized and survive because an IUD makes it harder for it to implant i the uterus. The progestin IUD may also thicken cervical mucus, making it more difficult for the sperm to enter the uterus. In addition, the progestin my keep women from ovulating.