"MECHANISM OF ACTION ? The precise mechanism of the contraceptive action of IUDs is not known; it is likely that several factors are involved (show table 1) [9-13]. Pregnancy appears to be prevented by a combination of a foreign body effect associated with the plastic or metal frame and through the specific action of any medication released (copper or levonorgestrel).

IUDs work primarily by preventing fertilization, and do not act as abortifacients (defined as interruption of an implanted pregnancy) [14]. When the uterus is exposed to a foreign body, a sterile inflammatory reaction occurs, which is toxic to sperm and ova and impairs implantation [11,13-16]. The production of cytotoxic peptides and activation of enzymes lead to inhibition of sperm motility, reduced sperm capacitation and survival, and sperm phagocytosis [17,18].

The primary effect of preventing fertilization is supported by three lines of evidence [11,13,15,19-22]:

• Sensitive assays for human chorionic gonadotropin (HCG) in IUD users did not detect transient "chemical" pregnancies.
• Tubal flushing studies did not find sperm or fertilized ova in the fallopian tubes of IUD users, but commonly found them in noncontraceptors.
• IUD users have lower rates of both intrauterine and tubal pregnancy than women not using contraception.