The mode of action of IUDs.

In women, IUDs produce alterations of the uterine environment in terms of a pronounced foreign body reaction. This biological response may interfere with steps of the reproductive process that normally take place before the ovum reaches the uterine cavity. In order to discuss this hypothesis on the mechanism of action of IUDs, this review is focussed on 1) detection in urine and blood of substances alleged to be specifically produced by the embryo, 2) migration of gametes in the female genital tract, and 3) microscopic features of ova recovered from the genital tract.