The IUD lost favor in the USA years ago because of several legal class-action suits charging serious damage/death to women from the effects of the IUD.  According to these lawsuits and medical reports, many women suffered Toxic Shock Syndrome which can -- or did -- lead to death.

There are many shapes and components of IUDs. The action still appears to be a "post-fertilization effect" abortive, in that they prevent a developing human being, already fertilized in the Fallopian tube, from attaching itself to the endometrium of the uterus.

"An intrauterine device (IUD) inserted into the uterus through the vagina and cervix usually interferes with implantation by causing a local inflammatory reaction. Some IUDs contain progesterone that is slowly released and interferes with the development of the endometrium so that implantation does not usually occur." (p. 58)(Keith Moore and T.V.N. Persaud, The Developing Human: Clinically Oriented Embryology, 6th ed., Philadelphia: W.B. Saunders Comp. 1998.

For research on the mechanism of the IUD, click here.

Intra-uterine devices, or IUDs, are used in China, forcibly inserted if necessary in unwilling women.

They do not provide a barrier to conception but rather cause a local inflammatory response which prevents implantation, again resulting in a "post-fertilization effect" which most likely prevents the ~100 cell embryo from implanting in the uterine lining..

According to a study reported in the Dec 2002 American Journal of Obstetrics and Gynecology ("Mechanisms of action of intrauterine devices: Update and estimation of postfertilization effects", by Stanford and Mikolaczyk, v 187, n6, 1699-1708):

"The possible mechanisms of action for the IUD in humans can each be classified as occurring before or after fertilization...Hormonal evidence indicates that the IUD does not generally inhibit ovulation in humans. The majority of women who are wearing hormonally active IUDs that reduce or eleminate menstruation still have ovulatory cycles as assessed by hormonal measurement and follicular ultrasonography.

"The possible postfertilization mechanisms of action of the IUD include the following: slowing or speeding the transport of the early embryo through the fallopian tube, damage to or destruction of the early embryo before it reaches the uterus, and prevention of implantation...It is well established that IUDs cause endometrial changes, with the type of changes present dependent on the type of IUD...

"The percentage of clinically recognized pregnancies that are ectopic [tubal] in users of inert and copper IUDs is about 3% to 4%, whereas for levonorgestrel and progesterone it is about 25%...These data suport the existence of a postfertilization effect for the IUD but cannot be used to estimate its magnitude...

"The Levonorgestrel-20 IUD (Mirena; Schering AG Pharmaceutical, Germany), developed in 1980...is highly effective to avoid clinical pregnancy, with a pregnancy rate around 0.1 per 100 woman-years. The levonorgestrel IUD has a minimal effect on the ovariam pituitary axis, and up to 85% of women are ovulatory during its use [i.e. these women still release an egg during each ovulation]. The rate of ovulation may increase with length of time that the device is worn.

"The strongest biologic effect of this IUD is local suppression of the endometrium...In addition, inflammation in the endometrium has bee demonstrated to be similar to that of inert IUDs. These endometrial effects result in decreased bleeding over time, and some women using the levonorgestrel IUD have amenorrhea [no bleeding]. However, amenorrhea does not necessarily imply that ovulation is not occurring but that it is primarily due to the endometrial effects...Cervical mucus favorable to the transport of sperm has been documented in the majority of ovulatory cycles during use of the levonorgestrel IUD.

"Overall, the consensus has been that levonorgestrel IUDs, like progesterone IUDs, act primarily by suppressing the endometrium, an effect that is likely to prevent implantation. However, endometrial effects may also result in the inhibition of sperm migration...

"With a clinical pregnancy rate of 0.1 per 100 women-years, the Levonorgestrel-20 IUD is thus estimated to be associated with the loss of 99.9% to 99.95% of all fertilized ova...

"In previous debates on the mechanism of action of the IUD, there was some discussion as to whether postfertilization effects were a "major" or "main" effect of the IUD. Our model illustrates clearly that, although the majority of pregnancy prevention occurs before fertilization, postfertilization effects make substantial and essential contributions to the effectiveness of all types of IUDs.

"With regard to the postfertilization effect of the IUD, it is likely that the majority of this effect occurs before the embryo enters the uterus.

"As discussed, the low recovery of ova from the uterus in IUD users, as well as the lack of hCG rise in more recent studies of IUD users, suggest that the major postfertilization effect is destruction of the early embryo in the Fallopian tube, in the same way that the major prefertilization effect is likely to be destruction of sperm and ova.

"For the copper IUD, this embryocidal effect may be more a result of inflammation and direct toxicity, whereas with the progestin IUDs it may result more from inhibition of transport through the Fallopian tube, along with prevention of implantation, preventing long-term viability of the embryo...

"We believe that these results have important implications for the counseling of women and couples who are considering the use of the IUD. Because our estimates are based on the best evidence currently available, we suggest that they could be used in clinical counseling for women who may object to postfertilization effects..."