IUDs Block Fertilization

IUDs achieve their primary contraceptive effect by interfering with sperm motility and survival.

Network: Winter 1996, Vol. 16, No. 2

Intrauterine devices (IUDs) achieve their primary contraceptive effect by interfering with sperm motility and survival. They also prevent fertilization and, in rare cases, implantation.

Any IUD prompts an endometrial reaction that promotes the release of leukocytes and locally-acting substances, called prostaglandins. These act simultaneously in the oviduct, cervix, uterine cavity and genital tract to impede sperm and egg development.

The presence of copper enhances this effect. Studies among women using copper IUDs have shown a reduced number of live sperm after intercourse when compared with nonusers. The interruption of sperm migration begins in the cervical mucus and continues in the uterus and oviducts. A California research team compared eight women 15 to 30 minutes after insemination. Sperm were detected in the oviducts of all four women not using IUDs, while no sperm were in the oviducts of copper IUD users.1 Many studies have shown copper to act as a spermicide. In a study of the Copper T 200, scientists observed that the sperm heads were detached from the tails in a majority of sperm cells.2

Fertilization is also interrupted through action on unfertilized eggs. A Chilean research team found few eggs in the tubes and uteruses of IUD-using women. They searched for ova by flushing the uterus on the second through the fifth day after ovulation. Eggs were found in one-third of 36 women using no contraceptives, while only one egg was recovered among 22 users of inert IUDs that contain no copper and none in 43 users of copper IUDs.3 A comparison of copper IUD users and women using no contraception showed that none of the eggs from copper IUD users were fully developed or fertilized, in contrast to more than half of the eggs recovered from noncontraceptive users.4 Other data show the copper IUD slows the development and transportation of eggs. When screened for a hormone secreted just prior to implantation of a fertilized egg, one study found few IUD users who tested positive for the hormone. The study concluded that the IUD's prevention of implantation is very rare.5

FHI trials of 10,000 women in 23 countries conclude the annual pregnancy rate for the Copper T 380 is very low, 0.5 per 100 women (one pregnancy among 200 users). Copper accounts for this high degree of contraceptive efficacy, according to Irving Sivin, senior scientist at the Population Council, which developed the Copper T 380. "With the inert IUD, we know that sperm transport is interfered with," says Sivin. "But with copper, it is interfered with more. And the more copper, the more interruption."

Copper IUDs also function well as emergency contraception when inserted within five days after unprotected intercourse. Emergency contraception is used after unprotected coitus to avoid pregnancy.

-- Sarah Keller


Footnotes

1. Tredway DR, Umezaki CU, Mishell DR, et al. Effect of intrauterine devices on sperm transport in the human being: Preliminary report. Am J Ob Gyn 1975;123(7):734.
2. Ortiz ME, Croxatto HB. The mode of action of IUDs. Contraception 1987; 36(1):44.
3. Ortiz, 55.
4. Alvarez F, Brache V, Fernandez E, et al. New insights on the mode of action of intrauterine contraceptive devices in women. Fertil Steril 1988;49(5):768-73.
5. 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):265-70.