The mode of action of IUDs.

ABSTRACT

PIP: This review delves into the mode of action of IUDs in greater detail than the commonly held theory that IUDs prevent implantation: it discusses whether IUDs affect fertilization, gamete migration or development of fertilized ova. In order to determine whether IUDs prevent fertilization, noninvasive methods of detecting fertilization, or very early pregnancy tests, would be necessary. Two approaches are to assay an alleged immunosuppressive early pregnancy factor, and to design extremely sensitive assays for trophoblastic gonadotrophin (hCG). In fertile cycles, such studies found a 6 to 57% incidence of fertilized ova that did not result in pregnancy. Comparable studies in IUD users sought a transient rise in hCG. Some researchers have seen a fleeting hCG with standard assays, but one laboratory using a new immunoradiometric assay found hCG in only 0.9% of cycles in IUD users. Following sperm or egg migration in women is possible by flushing the vagina and endocervix, or the tubes during surgery. Normally sperm can reach the oviduct in 2 hours and remain viable as long as 85 hours. With an IUD in place, several searches recovered no sperm in the tubes, presumably they were phagocytosed. Copper IUDs especially reduced numbers of sperm, and those found often had heads decapitated from tails. Ovum migration in IUD wearers was not appreciably affected through the oviduct, but few eggs were found in the uterus, again far fewer were found in copper IUD users. Looking at ova that were detected in IUD users, none were developing normally, the rest were classified as either abnormal or uncertain. Ova from copper IUD users were distinctive for being without vitellus and surrounded by macrophages. This preliminary research as a whole suggests that IUDs affect events prior to implantation, specifically ovum development in the tubes, sperm migration, and ovum transport in the uterus.