So how to explain Americans’ continued hesitation to embrace the IUD?
Certainly there are lingering doubts from the days of the Dalkon Shield, an IUD that was pulled off the market in 1974 because it caused bacterial infections. This contributed to Congress's decision to increase the FDA's authority to oversee medical devices. "It was a defective device that should never have been allowed on the market," says Anne Foster-Rosales, MD, chief medical officer for Planned Parenthood Golden Gate and assistant clinical professor at the University of California, San Francisco. "The old device had a string made from a material that allowed bacteria to cling to it and climb into the uterus and create bacterial infections. The strings and materials in modern IUDs are made of totally different materials and are considered extremely safe."
The two IUDs currently on the market are ParaGard, a copper IUD that lasts for 12 years and is considered 99.2% effective, and Mirena, which contains a synthetic hormone called levonorgestrel and retains its 99.9% effectiveness rate for five years. (Neither is effective at preventing sexually transmitted diseases, or STDs, just pregnancy.)
While the exact mechanism of action of IUDs is not completely understood, research points to specific reasons for the superior efficacy of ParaGard and Mirena. The copper IUD works primarily as a spermicide. The copper on the IUD reduces sperm motility such that sperm rarely reach the fallopian tube and are unable to fertilize the egg. The levonorgestrel-releasing IUD primarily works by causing cervical mucus to become thicker so that sperm cannot enter the uterus and tubes to fertilize the egg. Should the sperm manage to enter the uterus or the fallopian tubes, this IUD also creates a lining in the uterus that is unfriendly to fertilized eggs.
The copper IUD lasts longer but can cause some women to have heavier, more painful periods, so it may not be the best option for women who already have those issues. "It’s not dangerous, but it can be a nuisance," says Dr. Foster-Rosales, "so it’s important that women are aware of it and willing to deal with it."