The main mechanisms of action of IUDs occur before fertilization, by preventing sperm from ever reaching the egg. The specific mechanism for preventing sperm from reaching the egg differs by type of IUD.
Hormonal Hormonal IUDs (referred to as intrauterine systems in the UK) work by releasing a small amount of
levonorgestrel, a
progestin. The progestin released by hormonal IUDs primarily acts locally within the uterus, resulting in much lower systemic progestin levels than other
progestogen only contraceptives. Each type of hormonal IUD varies in size, amount of levonorgestrel released, and duration of effectiveness. The predominant mechanism of action of progestin in the uterus is thickening the cervical mucus to prevent sperm from reaching the fallopian tubes and ultimately the egg. Hormonal IUDs can also thin the endometrial (uterine) lining and potentially impair implantation, but this is not their usual function. Because they thin the endometrial lining, hormonal IUDs often reduce or entirely stop menstrual
bleeding. As a result, they are often used to treat
menorrhagia (heavy
menses), once pathologic causes of menorrhagia (such as
uterine polyps) have been ruled out.
Non-hormonal Copper Copper IUDs acts as a spermicide. Copper ions released from the IUD damage sperm and disrupt their ability to move, preventing sperm from traveling through the cervical mucus or destroying them as they pass through so that they never meet the egg. They also start a local inflammatory reaction that causes the uterus and fallopian tubes to produce a fluid that contains white blood cells, enzymes, and
prostaglandins, which are toxic to sperm. While not the main mechanism of action, studies have found that copper can also alter the endometrial (uterine) lining. This alteration can prevent implantation of a fertilized egg ("
blastocyst"), but it cannot disrupt a fertilized egg that has already been implanted in the uterine lining.
Inert Inert IUDs are not approved for use in the United States, the UK, or Canada. Inert IUDs do not have a
bioactive component. They are made of inert materials such as
stainless steel or
plastic. Their primary mechanism of action is to cause a local
foreign body reaction, which makes the uterine environment hostile to both
sperm and
implantation. They may have higher rates of preventing pregnancy fertilization, instead of before fertilization, compared to copper or hormonal IUDs. ==Types==