IUDs work primarily by preventing fertilization, not implantation. Both the American College of Obstetricians and Gynecologists (ACOG) and the World Health Organization state that the main mechanism of IUDs is stopping sperm from ever reaching or successfully joining with an egg. That said, the question of whether IUDs could also have secondary effects on implantation is more nuanced, and the answer differs slightly depending on the type of IUD.
How Copper IUDs Work
The copper IUD (sold as Paragard in the U.S.) contains no hormones. It works by releasing copper ions into the uterus, which creates an environment that is toxic to sperm. Copper affects both sperm function and movement, making it extremely difficult for sperm to reach the fallopian tubes where fertilization normally happens. The WHO specifically describes the copper IUD as preventing fertilization “by causing a chemical change in sperm and egg before they meet.”
Because copper IUDs don’t suppress ovulation, eggs are still released each cycle. The contraceptive effect depends almost entirely on disabling sperm before they can do their job. This is an important distinction: the copper IUD acts before fertilization rather than after it.
How Hormonal IUDs Work
Hormonal IUDs release a small amount of progestin directly into the uterus. Their primary mechanism is thickening the cervical mucus so much that sperm can’t pass through the cervix into the uterus. They also reduce sperm motility inside the reproductive tract, further impairing fertilization.
During the first year of use, hormonal IUDs can suppress ovulation in some cycles. After that first year, most women resume ovulating regularly, but the thickened cervical mucus continues to block sperm effectively. ACOG notes that the progestin also thins the uterine lining, which is a secondary effect worth understanding in the context of the implantation question.
The Implantation Question
This is where the conversation gets more complex. Both types of IUDs do cause changes to the uterine lining. Hormonal IUDs thin the endometrium as a known side effect of progestin (this is also why they reduce menstrual bleeding). Copper IUDs trigger a low-level inflammatory response in the uterine lining. In theory, either of these changes could make the uterus less hospitable to a fertilized egg trying to implant.
The key question is whether these endometrial changes actually matter in practice, given how effectively IUDs prevent fertilization in the first place. The available evidence suggests that fertilization rarely, if ever, occurs in IUD users, making the implantation question largely theoretical.
What Studies of Embryo Recovery Show
Researchers have tried to answer this question directly by looking for fertilized eggs in the reproductive tracts of women using IUDs. A systematic review published in The Linacre Quarterly examined multiple studies, including one particularly notable investigation by Alvarez and colleagues that compared IUD users to women not using contraception.
That study found no normally developing embryos in the IUD group, compared to ten embryos found in women using no contraception. A later reanalysis in 2007 suggested that two of the recovered specimens from IUD users might have been embryos, and two others were uncertain. Even under the most generous interpretation, this points to fertilization being rare in IUD users, not common.
The researchers concluded that while eggs can be found in the fallopian tubes of IUD users (confirming that ovulation still occurs), evidence of fertilization is scarce. The numbers are small enough that no definitive conclusion about post-fertilization effects can be drawn, but the data consistently points to pre-fertilization mechanisms doing the heavy lifting.
What Ectopic Pregnancies Tell Us
Some people wonder whether ectopic pregnancies in IUD users prove that fertilization is happening regularly, with the IUD only blocking implantation in the uterus. The data doesn’t support this interpretation. A meta-analysis of 16 case-control studies involving nearly 22,000 women found that current IUD use does not increase the overall risk of ectopic pregnancy compared to women who aren’t pregnant. When an IUD fails and pregnancy does occur, that pregnancy is more likely to be ectopic than a pregnancy without an IUD, but this reflects the IUD’s effectiveness at preventing uterine pregnancies rather than evidence that fertilization is happening frequently.
Why the Definition of Pregnancy Matters Here
Part of why this question generates so much discussion is that people define the start of pregnancy differently. The medical community, including ACOG, defines pregnancy as beginning at implantation, not fertilization. By this standard, anything that acts before implantation is contraception, not abortion. Federal policy has long aligned with this definition.
However, some religious traditions and some state laws define pregnancy as beginning at fertilization. Under that framework, even a theoretical post-fertilization effect would raise ethical concerns. This is a values-based distinction, not a scientific disagreement. The science itself is fairly consistent: IUDs work overwhelmingly by preventing sperm from reaching the egg. Whether a rare, unproven secondary effect on implantation matters to you depends on your personal ethical framework.
The Bottom Line on Mechanism
The best available evidence shows that IUDs prevent pregnancy by blocking fertilization. Copper IUDs impair sperm function. Hormonal IUDs thicken cervical mucus and reduce sperm motility. Both types do alter the uterine lining, but because fertilization is so effectively prevented upstream, the endometrial changes appear to play little to no practical role in how IUDs work. No study has definitively demonstrated that IUDs prevent the implantation of a normally developing embryo as a regular part of their contraceptive action.