For decades, clinicians and the public assumed that copper intrauterine devices (IUDs) prevented pregnancy by preventing implantation. There also was fear that IUDs could increase a woman’s risk of HIV infection. Results of a new study suggested these assumptions are incorrect.

“A key point of the paper is we always assumed, from the 1980s on, that the IUD was preventing implantation, but that’s never been proven,” says Karen Smith-McCune, MD, PhD, professor emeritus in the department of obstetrics, gynecology, and reproductive sciences at the University of California, San Francisco.

One theory was the egg could not be fertilized because of the spermicidal effects of the copper IUD. “But there’s been this dogma in the field that maybe the way these devices work is they prevent implantation of a fertilized egg, in which they would be like abortifacients,” Smith-McCune says.

The data suggested this is not true — at least for the copper IUD.1 “What has been shown is the egg quality and sperm viability are impaired,” she says. “But you can’t argue there is an effect on implantation, especially for the copper IUD, because it looks the same midcycle for any woman who was without contraception.” The study’s findings suggested that implantation could occur normally if the egg was fertilized, she adds.

According to the authors of the 2019-2020 edition of Managing Contraception, the copper IUD works primarily as a spermicide. “Copper ions inhibit sperm motility and acrosomal enzyme activation so that sperm rarely reach the fallopian tube and are unable to fertilize the ovum. The sterile inflammatory action created in the endometrium phagocytizes the sperm,” the authors wrote. “Experimental evidence suggests that the copper IUDs do not routinely work after fertilization. They are not abortifacients.”2

In the cross-sectional study, Smith-McCune and colleagues compared transcriptome from the endometrium or cervical transformation zone from four groups of women, including a control group that used no hormonal or intrauterine contraception. The other groups used levonorgestrel combined oral contraceptives, copper IUDs, and a levonorgestrel-releasing intrauterine system.1

Researchers pursued the study to investigate whether different contraceptives had different effects on HIV infection, Smith-McCune says.

In previous research, investigators found that women using Mirena, the levonorgestrel-releasing intrauterine system, experienced a huge inflammatory signal. “We didn’t know whether that was from the foreign body or hormone in the contraceptive,” she adds. “We compared these different molecular signals from the endometrium in those three contraceptive groups and a hormonal group.”

Investigators collected samples from women in their post-luteal phase. “Our hypothesis was the IUD groups would look the same: inflammation from having a foreign body,” Smith-McCune says. “The results were the opposite.”

Results from women using the copper IUD looked exactly like thos of the control group, she adds.

Women using a hormonal IUD experienced inflammation, indicating that the foreign body was not the pivotal mechanism by which the devices worked, Smith-McCune says.

“I think the results present a counterargument to resistance to the IUD,” she notes. “Policymakers who are resistant to IUD use can take our data as evidence that strongly suggests it is not preventing implantation.”

Researchers also were concerned about the inflammation making women more susceptible to HIV infection, but this proved untrue. “We did more studies on that, and we found there was no difference in infectivity itself with women using Mirena compared to women in controls,” Smith-McCune says. “We were relieved by that; women [who use IUDs] are not at more risk of HIV.”

The study’s takeaway message is the presence of a foreign body is not enough for contraceptive effectiveness. The release of copper causes sperm toxicity, the IUD’s main mechanism of action. The presence of a hormone in levonogestrel IUDs works by causing cervical mucus thickening, preventing sperm from entering the fallopian tubes.

“The overarching theory was that having this foreign body made IUDs work. Our results suggest this is unlikely to be correct,” Smith-McCune adds.


  1. Smith-McCune K, Thomas R, Averbach S, et al. Differential effects of the hormonal and copper intrauterine device on the endometrial transcriptome. Sci Rep 2020;10:6888.
  2. Hatcher RA, Haddad L, Zieman M, et al. Managing Contraception 2019-2020.15th ed. Bridging the Gap Foundation; 2019.