The major effect of all intrauterine devices (IUD) is to induce a local inflammatory reaction in the endometrium whose cellular and humoral components are released into the uterine cavity. This inflammatory reaction has a variable effect on the reproductive strategy of the species studied. For example, this foreign body reaction can be localized within the uterus of rodents; and in farm animals it can have striking extrauterine effects. Thus, the action of IUDs in humans cannot be discerned from animals. In humans, copper ions released from Cu-IUDs enhance the inflammatory response and reach concentrations in the luminal fluids of the genital tract that are toxic for spermatozoa and embryos. In women using the IUD, the entire genital tract seems affected, at least in part, because of luminal transmission of the fluids that accumulates in the uterine lumen. This affects the function or viability of gametes, decreasing the rate of fertilization and lowering the chances of survival of any embryo that may be formed, even before it reaches the uterus. Studies on the recovery of eggs from women using IUDs and from women not using contraception show that embryos are formed in the tubes of IUD users at a much lower rate compared with nonusers. This is believed to be the major action of IUDs. Therefore, the common belief that the major mechanism of action of IUDs in women is through destruction of embryos in the uterus (i.e., abortion) is not supported by the available evidence. In Cu-IUD users, it is likely that few spermatozoa reach the distal segment of the fallopian tube, those that encounter an egg may be in poor condition. Thus, the few eggs that are fertilized have little chance for development and their possibility for survival in the altered tubal milieu become worse as they approach the uterine cavity.
PIP: All IUDs induce a local inflammatory reaction that disturbs the functioning of the endometrium and myometrium and changes the microenvironment of the uterine cavity. Moreover, these effects alter signaling between uterus and ovary. The entire genital tract seems affected, at least in part because of luminal transmission of fluids accumulating in the uterine lumen. Copper or progesterone-releasing IUDs may attenuate or accentuate the inflammatory response, disturb the physiology of the gametes in the female genital tract, or destroy the viability of the embryos or endometrial receptivity to implantation. Studies on the recovery of eggs reveal that embryos are formed in the tubes of IUD users at a significantly lower rate compared to non-users. This, rather than the destruction of embryos in the uterus, appears to be the IUD's major mechanism of action.