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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