An intrauterine device (IUD) is a highly effective form of birth control, a small, T-shaped device placed inside the uterus to prevent pregnancy. A common concern is whether an IUD can directly cause a miscarriage. This article explores how IUDs work, the rare instances of pregnancy with an IUD, and the associated risks.
How IUDs Prevent Conception
IUDs prevent pregnancy through distinct mechanisms depending on whether they are hormonal or non-hormonal. Hormonal IUDs release a small amount of progestin, a synthetic form of progesterone, into the uterus. This progestin primarily thickens cervical mucus, creating a barrier that blocks sperm from reaching an egg. Hormonal IUDs can also thin the uterine lining and sometimes suppress ovulation, preventing the release of an egg altogether.
Conversely, non-hormonal copper IUDs prevent pregnancy without hormones. The copper releases ions toxic to sperm, impairing their movement and ability to fertilize an egg. This copper also creates an inflammatory reaction within the uterus, making the environment inhospitable for sperm and potentially for implantation. Both types of IUDs are highly effective, with pregnancy rates of less than 1% per year.
Pregnancy With an IUD in Place
While IUDs are over 99% effective, conception can rarely occur. If pregnancy happens with an IUD in the uterus, there is an increased risk of complications. One risk is ectopic pregnancy, where the fertilized egg implants outside the uterus. Although IUDs reduce the overall risk of ectopic pregnancy compared to not using contraception, a pregnancy with an IUD is more likely to be ectopic.
For pregnancies that implant inside the uterus despite an IUD, the device increases the risk of miscarriage. This includes a higher chance of spontaneous miscarriage, and even septic miscarriage, a severe infection. The risk of miscarriage can be as high as 40% to 50% if the IUD remains in place.
Managing Pregnancy When an IUD is Present
If pregnancy is confirmed with an IUD in place, healthcare providers generally recommend removing the device if its strings are visible and it can be safely retrieved. This removal is ideally performed during the first trimester to reduce complications. Removing the IUD can lower the risk of spontaneous miscarriage, preterm delivery, and infection. Though removal carries a small miscarriage risk, it is often outweighed by the risks of leaving the IUD in place.
If the IUD strings are not visible or the device cannot be easily removed, it may be left in place, but the pregnancy will require close monitoring. Continuing a pregnancy with a retained IUD carries elevated risks of complications, including late miscarriage, preterm labor, and intrauterine infection. Despite these risks, IUDs do not appear to increase the risk of birth defects in the fetus.
Clarifying the Miscarriage Question
IUDs do not directly cause miscarriages in the way a genetic abnormality or direct trauma might. Instead, their primary function is to prevent pregnancy from occurring by interfering with sperm function or fertilization. If, in rare cases, a pregnancy does occur despite the IUD’s presence, the device then becomes a foreign object within the uterus. Its continued presence can increase the likelihood of complications such as miscarriage or ectopic pregnancy.
The mechanism of action for IUDs primarily involves preventing fertilization or, less commonly, implantation. Some perspectives consider the prevention of implantation as an abortifacient effect. However, the scientific consensus emphasizes that IUDs mainly work by preventing sperm from reaching the egg or by creating a uterine environment inhospitable to fertilization, meaning their action occurs before a pregnancy is established. Therefore, an IUD does not typically “cause” a miscarriage in a previously established, healthy pregnancy, but rather increases the risk of pregnancy loss if it fails to prevent conception.