Intrauterine devices (IUDs) prevent fertilization primarily by interfering with the ability of sperm to survive and to ascend the fallopian tubes, where fertilization occurs.
Having a foreign body in the uterus, such as an IUD, causes both anatomical and biochemical changes that appear to be toxic to sperm. Studies have generally found that sperm are not as viable among IUD users, compared to other women.1
Particularly in the presence of copper-bearing devices, sperm have been absent or few in number in the upper female genital tract, concluded a report of a World Health Organization study group. "Spermatozoa can migrate to the fallopian tubes in some cases but are less likely to reach the normal site of fertilization."2 Scientists in Chile and the United States reached similar conclusions in their 1996 review of me
chanism of action research.3
When a foreign body is in the uterus, the endometrium reacts by releasing white blood cells, enzymes and prostaglandins; and these reactions of the endometrium appear to prevent sperm from reaching the fallopian tubes. In addition, copper-bearing IUDs release copper ions into the fluids of the uterus and the fallopian tubes, enhancing the debilitating effect on sperm.
Evidence for these mechanisms includes physical examination of women's eggs. When an ovum is fertilized, it begins to produce human chorionic gonadotropin (hCG) near the time of implantation. A 1987 study to monitor hCG production in 40 women using IUDs found only one probable fertilized egg among 107 cycles. "Whatever the IUD's specific mechanism of action, it appears that the IUD effectively interrupts the reproductive process before implantation," the study concluded.4
Another way researchers have documented the IUD's mechanism of action is to recover an ovum during a woman's fertile period. Using this approach, researchers recovered ova from 115 women using no contraception and 56 women using IUDs.
Half of the women using no contraception who had intercourse during the fertile period had ova that were consistent in appearance with fertilized eggs. In contrast, none of the ova taken from copper IUD users who had intercourse appeared to be fertilized. Also, no ova were found in the uterus of any of the copper IUD users. "IUDs exert effects that extend beyond the body of the uterus and interfere with steps of the reproductive process that take place before the eggs reach the uterine cavity," concluded Dr. Frank Alvarez and colleagues.5
The levonorgestrel IUD, called an intrauterine system, uses different mechanisms. Like other progestin methods, this device prevents pregnancy primarily by thickening cervical mucus, which inhibits the ability of sperm to enter the uterus.
-- William R. Finger
References
- Ortiz ME, Croxatto H. The mode of action of IUDs. Contraception 1987;36(1):37-53.
- World Health Organization. Mechanism of Action, Safety and Efficacy of Intrauterine Devices -- Report of a Scientific Committee, WHO Technical Report Series 753. (Geneva: World Health Organization, 1987)16.
- Ortiz ME, Croxatto HB, Bardin CW. Mechanisms of action of intrauterine devices. Obstet Gynecol Surv 1996;51(12):S42-S51.
- Wilcox AJ, Weinberg CR, Armstrong EG, et al. Urinary human chorionic gonadotropin among intrauterine device users: detection with a highly specific and sensitive assay. Fertil Steril 1987;47(2):265-69.
- Alvarez F, Brache V, Fernández E, et al. New insights on the mode of action of intrauterine contraceptive devices in women. Fertil Steril 1988;49(5):768-73.
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