Final answer:
The primary contraceptive action of an IUD is blocking fertilization, achieved by thickening cervical mucus and, depending on the type, potentially preventing ovulation or implantation.
Step-by-step explanation:
The primary contraceptive action of an intrauterine device (IUD) is blocking fertilization. This is achieved through several mechanisms depending on the type of IUD. Hormonal IUDs, such as the brand Mirena, release progestin which thickens the cervical mucus making it difficult for sperm to reach the egg, and may also prevent ovulation. The copper in copper IUDs interferes with the movement of sperm and may prevent implantation if fertilization does occur. Both types of IUDs are inserted into the uterus and may remain in place for an extended period of time, providing reliable and reversible contraception.